technical · sourced answer

How to Prevent Email Spoofing: SPF, DKIM, and DMARC Guide

To prevent email spoofing, you must implement a combination of SPF, DKIM, and DMARC DNS records. These protocols verify that the sending server is authorized to send mail on behalf of your domain and ensure the message content has not been altered in transit.

Implement SPF Records

Sender Policy Framework (SPF) is a DNS record that lists all IP addresses and domains authorized to send email for your domain. When a receiving server gets an email, it checks the SPF record of the domain in the From address. If the sending IP is not listed, the email may be marked as spam or rejected. Use the SendHQ free tools (https://sendhq.cc/tools) to validate your SPF syntax and ensure no unauthorized IPs are included.

Configure DKIM Signing

DomainKeys Identified Mail (DKIM) adds a cryptographic signature to the email header. The sending server signs the message with a private key, and the receiving server verifies this signature using a public key published in your DNS records. This prevents attackers from altering the email content during transit and proves the email actually originated from your infrastructure.

Deploy DMARC Policies

Domain-based Message Authentication, Reporting, and Conformance (DMARC) ties SPF and DKIM together. It tells receiving servers what to do if an email fails SPF or DKIM checks. You can set a policy of none (monitor), quarantine (send to spam), or reject (block entirely). DMARC also provides reporting back to the domain owner to identify spoofing attempts and configuration errors.

Maintain DNS Hygiene

Preventing spoofing requires ongoing maintenance of DNS records. Regularly audit your SPF record to remove old third party senders and rotate DKIM keys periodically to maintain security. Ensure that your DMARC policy eventually moves from none to reject to fully stop spoofed emails from reaching user inboxes.

Use Secure SMTP Extensions

Beyond DNS records, use STARTTLS to encrypt the connection between mail servers. This prevents man in the middle attacks from intercepting credentials or modifying mail in transit. Implementing SMTP authentication ensures that only authorized users can relay mail through your server, reducing the risk of internal spoofing.

Questions teams ask

What is the difference between SPF and DKIM?

SPF authorizes specific servers to send mail via DNS lists, while DKIM uses cryptographic signatures to verify the sender and ensure message integrity.

Does DMARC replace SPF and DKIM?

No, DMARC relies on SPF and DKIM. It provides the policy instructions on how to handle emails that fail those two authentication checks.

Can I use a p=reject policy immediately?

It is recommended to start with p=none to monitor traffic and ensure legitimate mail is not blocked before moving to p=quarantine and finally p=reject.

What happens if I have multiple SPF records?

Having multiple SPF records is a syntax error and will cause SPF to fail. You must merge all authorized senders into a single TXT record.

Primary sources

Continue learning

email dkim spf dmarcspf record syntaxdmarc checkdkim setupemail domain verification