---
title: Email questions and glossary terms | SendHQ
description: Direct, sourced answers to email infrastructure questions plus definitions for SPF, DKIM, DMARC, MX, SMTP, bounces and deliverability terms.
canonical: https://sendhq.cc/answers
last-updated: 2026-09-22
---
# Email questions, answered directly

Question-and-answer pages for transactional email, domain authentication, deliverability and agent email workflows, plus a glossary of email infrastructure terms.

- [Responsive Email Template: Free HTML Code That Works](https://sendhq.cc/guides/email-responsive-design-template): A free fluid-hybrid responsive email template that works in Gmail, Outlook, and Apple Mail, plus why media queries fail and the best practices to follow. Read answer →
- [Domain Warm-Up: Schedule, Timeline, and Metrics](https://sendhq.cc/guides/domain-warm-up): How to warm up a new email domain: a 1.5x-per-stage schedule, how long it takes (4-8 weeks), who to email first, and the bounce and complaint limits to watch. Read answer →
- [Preheader in Email: What It Is, Length, and HTML Code](https://sendhq.cc/terms/pre-header-in-email): A preheader is the preview text shown after the subject line. Ideal length (under 90 characters), the hidden-div HTML with padding, and examples that work. Read answer →
- [Spam Terms in Email: Which Words Filters Really Score](https://sendhq.cc/guides/spam-terms-email): Spam trigger words with real SpamAssassin scores: 'Dear Friend' +2.6, '100% guaranteed' +2.7, 'act now' +0.1. What to avoid, what Gmail checks, and how to test. Read answer →
- [Email for a Subdomain: DNS Setup for Sending and Receiving](https://sendhq.cc/guides/email-for-a-subdomain): Set up email on a subdomain: DKIM, SPF and MX records it needs, how DMARC sp= applies, receiving mail, and splitting transactional from marketing. Read answer →
- [Mailto in PHP: Build Mailto Links and Send Email](https://sendhq.cc/guides/mailto-php-send-email): Build RFC 6068 mailto links in PHP (%20 not +, %0D%0A line breaks), and send email with mail(), PHPMailer 7, or an HTTP API without landing in spam. Read answer →
- [SpamAssassin Scores: What They Mean and How to Fix Them](https://sendhq.cc/guides/spamassassin-scores): SpamAssassin scores explained with real rule points from the 4.0 rule set: the 5.0 threshold, score sets, the rules that hurt senders, and how to test locally. Read answer →
- [Google Postmaster Tools: Setup and Dashboards Guide](https://sendhq.cc/guides/google-postmaster-tools-guide): Set up Google Postmaster Tools, read the spam rate and compliance dashboards correctly, add Feedback-ID, and use the v2beta API. Gmail-only data, explained. Read answer →
- [What Are ESPs? Email Service Providers Explained](https://sendhq.cc/terms/what-are-esps): ESP means email service provider: the service that sends your email. The 4 types, current pricing, Gmail and Yahoo rules, and how to choose one. Read answer →
- [Noreply Email Address: What It Is and When to Use It](https://sendhq.cc/terms/noreply-email-address): A noreply address is an unmonitored sender. What happens to replies (550 5.1.1, null MX 556), the deliverability myth, and better Reply-To setups. Read answer →
- [What Are OTP Codes? How One-Time Passwords Work](https://sendhq.cc/terms/what-are-otp-codes): OTP codes are single-use passwords. How HOTP and TOTP work, what NIST SP 800-63B-4 says about email OTP, and the verifier rules that stop guessing. Read answer →
- [What Port Does TLS Use?](https://sendhq.cc/terms/what-port-does-tls-use): TLS has no single port. Learn how application protocols choose ports, how implicit TLS differs from STARTTLS, and which SMTP port to configure. Read answer →
- [How Do Resend Webhooks Work?](https://sendhq.cc/terms/how-do-resend-webhooks-work): Learn the Resend webhook workflow, from event delivery and signature verification to idempotent processing, retries, replays, and deployment. Read answer →
- [Postmark Alternative: Technical Comparison for Product Teams](https://sendhq.cc/guides/postmark-alternative): Compare Postmark alternatives including Amazon SES, Resend, and SendHQ. Technical evaluation of pricing, API features, and AI agent readiness. Read answer →
- [Mailgun Alternative: Technical Comparison for Product Teams](https://sendhq.cc/guides/mailgun-alternative): Compare Mailgun alternatives including Amazon SES, Resend, and SendGrid. Technical evaluation of pricing, API features, and agent readiness for developers. Read answer →
- [SendGrid Alternative: Technical Comparison for Product Teams](https://sendhq.cc/guides/sendgrid-alternative): Compare SendGrid alternatives including Amazon SES, Resend, Mailgun, and Postmark based on September 2026 pricing and technical capabilities. Read answer →
- [Best SES Alternative for Product Teams and AI Agents](https://sendhq.cc/guides/ses-alternative): Compare Amazon SES with Resend, SendGrid, Mailgun, and Postmark. Technical evaluation of pricing, API features, and agent readiness for developers. Read answer →
- [Choosing the Best Email API for SaaS: Technical Comparison](https://sendhq.cc/guides/email-api-for-saas): Compare transactional email APIs for SaaS based on verified September 2026 pricing and technical features for product teams and AI agents. Read answer →
- [Cheap Transactional Email API Comparison for Developers](https://sendhq.cc/guides/cheap-transactional-email-api): Compare transactional email API pricing and features. Technical evaluation of Amazon SES, Resend, SendGrid, Mailgun, and Postmark for cost efficiency. Read answer →
- [Email Verification Service Comparison for Product Teams](https://sendhq.cc/guides/email-verification-service): Compare email verification and transactional sending services. Technical analysis of pricing and features for developers and AI agent product teams. Read answer →
- [SES service AWS Evaluation: Pricing and Technical Comparison](https://sendhq.cc/guides/aws-ses-service): Compare AWS SES pricing and features against Resend, SendGrid, Mailgun, and Postmark. Technical evaluation for product teams and AI agents. Read answer →
- [What is Email Authentication: SPF, DKIM, and DMARC Explained](https://sendhq.cc/terms/what-is-email-authentication): Technical overview of email authentication protocols including SPF, DKIM, and DMARC to prevent spoofing and improve sender reputation. Read answer →
- [How to Read DMARC Aggregate Reports](https://sendhq.cc/terms/how-to-read-dmarc-reports): Learn how to parse and analyze DMARC aggregate reports to identify SPF and DKIM failures and improve email authentication alignment. Read answer →
- [What Port Does IMAP Use? Technical Port Guide](https://sendhq.cc/terms/what-port-does-imap-use): Technical guide on IMAP port assignments for non-encrypted and encrypted connections including IMAPS and STARTTLS configurations. Read answer →
- [Effective Follow Up Email Subject Lines for Developers](https://sendhq.cc/terms/follow-up-email-subject-lines): Learn how to write high conversion follow up email subject lines using technical patterns and best practices for professional communication. Read answer →
- [Effective Sales Email Subject Lines for Developers](https://sendhq.cc/terms/sales-email-subject-lines): Learn how to write high-conversion sales email subject lines using technical principles, character limits, and RFC standards for maximum visibility. Read answer →
- [DKIM Fail Body Hash Did Not Verify Diagnostic](https://sendhq.cc/terms/dkim-body-hash-did-not-verify): Technical guide to diagnosing and fixing DKIM body hash verification failures caused by message modification during transit. Read answer →
- [How to Check if an Email Domain is Valid](https://sendhq.cc/terms/how-to-check-if-an-email-domain-is-valid): Learn how to verify email domain validity using DNS lookups for MX records and syntax validation to ensure mail can be routed to the destination. Read answer →
- [How to Verify Email Address Without Sending Email](https://sendhq.cc/terms/how-to-verify-an-email-address-without-sending-email): Learn the technical methods for verifying email addresses without sending messages, including syntax checks, DNS lookups, and SMTP handshakes. Read answer →
- [Why are my emails going to junk? Diagnostic Guide](https://sendhq.cc/terms/why-are-my-emails-going-to-junk): Technical diagnostic guide for developers to identify and fix email deliverability issues causing messages to land in junk or spam folders. Read answer →
- [Why DMARC Says Quarantine or Reject Policy Not Enabled](https://sendhq.cc/terms/why-does-dmarc-say-policy-not-enabled): Learn why your DMARC record is flagged as not having a quarantine or reject policy and how to update your p= tag for enforcement. Read answer →
- [How to Prevent Email Spoofing: SPF, DKIM, and DMARC Guide](https://sendhq.cc/terms/how-to-prevent-email-spoofing): Learn how to prevent email spoofing by implementing SPF, DKIM, and DMARC records to authenticate your domain and protect your sender reputation. Read answer →
- [What is SendGrid Sandbox Mode?](https://sendhq.cc/terms/what-is-sendgrid-sandbox-mode): Technical explanation of SendGrid sandbox mode, how it restricts email delivery for testing, and how to transition to production mode. Read answer →
- [How to Get a Resend API Key for Email Sending](https://sendhq.cc/terms/how-to-get-a-resend-api-key): Step by step technical guide on generating and managing your Resend API key to integrate transactional email into your application. Read answer →
- [What is the AWS SES Endpoint?](https://sendhq.cc/terms/what-is-the-aws-ses-endpoint): Find the correct AWS SES endpoints for API requests and SMTP configurations based on your AWS region and connection method. Read answer →
- [How to Check if Port 587 is Open](https://sendhq.cc/terms/how-to-check-if-port-587-is-open): Learn how to verify if port 587 is open using telnet, nc, or PowerShell to troubleshoot SMTP submission and STARTTLS connectivity issues. Read answer →
- [SMTP Port 587 vs 25: Which Port to Use for Email Sending](https://sendhq.cc/terms/smtp-port-587-vs-25): Technical comparison of SMTP ports 25 and 587. Learn why port 587 is the standard for client submission and port 25 is for server relay. Read answer →
- [What is Port 587 Used For? SMTP Submission Port Explained](https://sendhq.cc/terms/what-is-port-587-used-for): Technical explanation of SMTP port 587, its role in email submission, the use of STARTTLS, and how it differs from ports 25 and 465. Read answer →
- [Is SMTP TCP or UDP? Technical Protocol Analysis](https://sendhq.cc/terms/is-smtp-tcp-or-udp): Technical explanation of why SMTP uses TCP instead of UDP for email transmission, including port assignments and reliability requirements. Read answer →
- [What is SMTPS Port 465?](https://sendhq.cc/terms/what-is-smtps): Technical explanation of SMTPS port 465, its use for implicit TLS encryption, and how it differs from STARTTLS on port 587. Read answer →
- [What Port Does SMTP Use? Standard SMTP Port Guide](https://sendhq.cc/terms/what-port-does-smtp-use): Technical guide on SMTP ports 25, 465, and 587. Learn which port to use for mail submission, relaying, and secure TLS connections. Read answer →
- [SMTP Server: Definition and Technical Overview](https://sendhq.cc/terms/smtp-server): Technical explanation of Simple Mail Transfer Protocol servers, their role in email routing, and common port configurations for developers. Read answer →
- [Webhook Signature Verification](https://sendhq.cc/terms/webhook-signature): Learn how webhook signature verification ensures the authenticity of incoming HTTP requests from email service providers to your server. Read answer →
- [Exponential Backoff Retries](https://sendhq.cc/terms/exponential-backoff): Learn how exponential backoff retries prevent server overload by increasing wait times between failed email delivery attempts. Read answer →
- [What is a Mail User Agent (MUA)?](https://sendhq.cc/terms/mail-user-agent): Technical definition of a Mail User Agent (MUA), its role in the email delivery chain, and how it interacts with MTAs and mail protocols. Read answer →
- [Custom Email Headers: Definition and Technical Implementation](https://sendhq.cc/terms/custom-email-header): Learn how custom email headers function in the SMTP protocol to pass metadata, track messages, and integrate with email delivery services. Read answer →
- [SMTP Authentication: Definition and Technical Implementation](https://sendhq.cc/terms/smtp-authentication): Technical overview of SMTP authentication, explaining how mail servers verify sender identity to prevent unauthorized relaying and spam. Read answer →
- [IP and Domain Warmup for Email Deliverability](https://sendhq.cc/terms/ip-warmup): Learn how IP and domain warmup establishes sender reputation by gradually increasing email volume to avoid spam filters and improve delivery rates. Read answer →
- [What is a Spam Trap? Definition and Impact](https://sendhq.cc/terms/spamtrap): Learn how spam traps work, the difference between pristine and recycled traps, and how they impact your sender reputation and deliverability. Read answer →
- [Greylisting: Email Spam Prevention Mechanism](https://sendhq.cc/terms/greylisting): Technical explanation of greylisting, how it delays email delivery to filter spam, and its impact on SMTP transmission and sender reputation. Read answer →
- [What is an Email Alias?](https://sendhq.cc/terms/email-alias): Technical definition of an email alias, explaining how multiple email addresses map to a single primary mailbox for routing and organization. Read answer →
- [DKIM Signature Header Explained](https://sendhq.cc/terms/dkim-signature): Technical overview of the DKIM-Signature header, explaining how cryptographic signatures verify email integrity and sender identity. Read answer →
- [List-Unsubscribe Header Definition and Implementation](https://sendhq.cc/terms/list-unsubscribe): Technical guide to the List-Unsubscribe email header, explaining its mechanical function, implementation methods, and impact on sender reputation. Read answer →
- [Verification Email: Definition and Technical Implementation](https://sendhq.cc/terms/verification-email): Technical overview of verification emails, including the double opt-in mechanism, security benefits, and implementation requirements for senders. Read answer →
- [ESMTP Extensions: Extended Simple Mail Transfer Protocol](https://sendhq.cc/terms/esmtp): Technical overview of ESMTP extensions, explaining how the EHLO command enables advanced mail features like STARTTLS and SMTP AUTH. Read answer →
- [Email Verification Definition and Process](https://sendhq.cc/terms/email-verification): Technical overview of email verification processes used to validate email address syntax and domain configuration before sending. Read answer →
- [Email Address Validation](https://sendhq.cc/terms/email-validation): Technical overview of email address validation, covering syntax checks, DNS verification, and the impact on sender reputation. Read answer →
- [DNS TXT Record: Definition and Email Use Cases](https://sendhq.cc/terms/txt-record): Learn how DNS TXT records function as text based entries for domain verification and email authentication protocols like SPF, DKIM, and DMARC. Read answer →
- [What is a DKIM Selector?](https://sendhq.cc/terms/dkim-selector): Learn how DKIM selectors allow email senders to use multiple public keys for a single domain to ensure secure email authentication and verification. Read answer →
- [Email Spoofing: Definition and Technical Mechanisms](https://sendhq.cc/terms/email-spoofing): Technical explanation of email spoofing, how SMTP allows header manipulation, and the authentication protocols used to prevent sender impersonation. Read answer →
- [Open Relay: Definition and Security Risks](https://sendhq.cc/terms/open-relay): Learn what an open relay is, how it allows unauthorized email sending, and why it leads to IP blacklisting and severe deliverability issues. Read answer →
- [SMTPS implicit TLS](https://sendhq.cc/terms/smtps): Technical overview of SMTPS implicit TLS, its operation on port 465, and how it differs from STARTTLS for secure email transmission. Read answer →
- [STARTTLS: Secure SMTP Communication via Opportunistic TLS](https://sendhq.cc/terms/starttls): Technical guide to STARTTLS, the SMTP extension that upgrades a plain text connection to a secure TLS encrypted session for email transmission. Read answer →
- [Dedicated IP vs Shared IP for Email Sending](https://sendhq.cc/terms/dedicated-ip): Technical comparison of dedicated and shared IP addresses for SMTP sending, focusing on reputation management and deliverability impact. Read answer →
- [WP Mail SMTP Definition and Functionality](https://sendhq.cc/terms/wp-mail-smtp): Technical overview of WP Mail SMTP, explaining how it replaces the default WordPress PHP mail function with professional SMTP relay services. Read answer →
- [Email Blocklist: Definition and Technical Overview](https://sendhq.cc/terms/email-blocklist): Technical explanation of email blocklists, including how they function, the impact on sender reputation, and methods for remediation and prevention. Read answer →
- [Notification Email: Definition and Technical Implementation](https://sendhq.cc/terms/notification-email): Technical overview of notification emails, focusing on transactional triggers, SMTP delivery, and authentication requirements for automated messaging. Read answer →
- [Email Subject Line](https://sendhq.cc/terms/email-subject-line): Technical definition and operational role of the email subject line within the SMTP message header and its impact on recipient interaction. Read answer →
- [Sender Reputation: Definition and Technical Impact](https://sendhq.cc/terms/sender-reputation): Technical overview of sender reputation, explaining how ISPs track sending behavior and the impact of IP and domain reputation on email delivery. Read answer →
- [Double Opt In: Definition and Implementation](https://sendhq.cc/terms/double-opt-in): Learn how double opt in works to verify email addresses, reduce spam complaints, and improve list hygiene for better deliverability. Read answer →
- [Email API Rate Limiting Explained](https://sendhq.cc/terms/email-rate-limiting): Technical guide to email API rate limiting, covering request quotas, HTTP 429 errors, and strategies for managing high volume transactional email traffic. Read answer →
- [Idempotency Key: Preventing Duplicate Email Sends](https://sendhq.cc/terms/idempotency-key): Learn how idempotency keys prevent duplicate email delivery during network retries and API timeouts in transactional email systems. Read answer →
- [Email Webhook: Definition and Technical Implementation](https://sendhq.cc/terms/email-webhook): Technical guide to email webhooks, explaining how HTTP callbacks notify applications of email delivery events like bounces, opens, and clicks. Read answer →
- [What is a Spam Complaint in Email Delivery?](https://sendhq.cc/terms/spam-complaint): Technical explanation of spam complaints, feedback loops, and the impact of user reported spam on sender reputation and delivery rates. Read answer →
- [Hard Bounce vs Soft Bounce: Technical Definitions](https://sendhq.cc/terms/hard-bounce): Technical breakdown of hard vs soft email bounces, including SMTP response codes, delivery failure causes, and impact on sender reputation. Read answer →
- [Email Suppression List Definition and Implementation](https://sendhq.cc/terms/suppression-list): Learn how email suppression lists prevent sending to invalid or opted out addresses to protect sender reputation and ensure compliance. Read answer →
- [Authenticated Received Chain (ARC) Explained](https://sendhq.cc/terms/arc): Learn how Authenticated Received Chain (ARC) preserves email authentication results during forwarding to prevent DMARC failures. Read answer →
- [Abuse Reporting Format (ARF) Explained](https://sendhq.cc/terms/arf): Technical overview of the Abuse Reporting Format (ARF) used by mailbox providers to notify senders when users mark emails as spam. Read answer →
- [Email Feedback Loop Definition and Implementation](https://sendhq.cc/terms/feedback-loop): Learn how email feedback loops work to notify senders when recipients mark messages as spam, helping maintain sender reputation and deliverability. Read answer →
- [Verified Mark Certificate (VMC) Explained](https://sendhq.cc/terms/vmc): Learn about Verified Mark Certificates (VMC), their role in BIMI, and how they enable brand logos in email clients. Read answer →
- [BIMI: Brand Indicators for Message Identification](https://sendhq.cc/terms/bimi): Learn about Brand Indicators for Message Identification (BIMI), the standard for displaying brand logos in email clients via DMARC and SVG files. Read answer →
- [Email Headers: Technical Definition and Structure](https://sendhq.cc/terms/email-header): Technical guide to email headers, explaining their structure, role in routing, and importance for authentication and deliverability. Read answer →
- [SMTP Envelope vs Headers: Understanding the Difference](https://sendhq.cc/terms/email-envelope): Learn the technical distinction between the SMTP envelope and email headers, including how they affect routing, SPF, and DMARC alignment. Read answer →
- [Return-Path Header Explained](https://sendhq.cc/terms/return-path): Technical overview of the Return-Path header, its role in bounce handling, and its critical importance for SPF authentication and email deliverability. Read answer →
- [Message-ID header definition and function](https://sendhq.cc/terms/message-id): Technical overview of the Message-ID header, its role in email threading, uniqueness requirements, and impact on deliverability. Read answer →
- [MIME: Multipurpose Internet Mail Extensions](https://sendhq.cc/terms/mime): Learn how Multipurpose Internet Mail Extensions (MIME) allow email to support non-ASCII text, attachments, and HTML content. Read answer →
- [What is POP3? Post Office Protocol Version 3 Explained](https://sendhq.cc/terms/pop3): Technical overview of Post Office Protocol Version 3 (POP3), explaining its mechanism for downloading emails from a server to a local client. Read answer →
- [IMAP: Internet Message Access Protocol Explained](https://sendhq.cc/terms/imap): Technical overview of the Internet Message Access Protocol (IMAP), explaining how it synchronizes email across multiple devices and differs from POP3. Read answer →
- [What is an MX Record? Definition and Function](https://sendhq.cc/terms/mx-record): Learn how Mail Exchanger (MX) records route incoming email to the correct mail server using DNS priority and hostname mapping. Read answer →
- [DMARC Alignment Relaxed vs Strict](https://sendhq.cc/terms/dmarc-alignment): Learn the technical difference between relaxed and strict DMARC alignment for SPF and DKIM to ensure proper email authentication and delivery. Read answer →
- [DMARC Record: Definition and Implementation Guide](https://sendhq.cc/terms/dmarc-record): Learn how DMARC records use SPF and DKIM to prevent email spoofing and provide reporting on domain authentication status. Read answer →
- [DMARC: Domain-based Message Authentication, Reporting and Conformance](https://sendhq.cc/terms/dmarc): Technical guide to DMARC, explaining how it leverages SPF and DKIM to prevent domain spoofing and provide authentication reporting for senders. Read answer →
- [DKIM DomainKeys Identified Mail Explained](https://sendhq.cc/terms/dkim): Technical overview of DomainKeys Identified Mail (DKIM), covering cryptographic signing, DNS verification, and its role in email authentication. Read answer →
- [SPF Sender Policy Framework Explained](https://sendhq.cc/terms/spf): Technical guide to Sender Policy Framework (SPF), explaining DNS record syntax, verification mechanics, and its role in email authentication. Read answer →
- [What is a Mail Transfer Agent (MTA)?](https://sendhq.cc/terms/mail-transfer-agent): Technical definition of a Mail Transfer Agent (MTA), explaining its role in routing email via SMTP from sender to recipient. Read answer →