Domain Warm-Up: Schedule, Timeline, and Metrics

What does warming up a sender mean?

Domain warm-up is the practice of sending a small, slowly increasing volume of email from a new sending domain or subdomain so that mailbox providers such as Gmail, Microsoft Outlook.com, and Yahoo can build a reputation for it before you send at full volume. A domain with no sending history that suddenly sends 50,000 messages looks like a spammer who just registered it, and receivers respond with deferrals (4xx SMTP responses), spam-folder placement, or outright blocks. Warm-up exists because reputation is earned from recipient behavior: low complaint and bounce rates, opens, replies, and messages moved out of spam. Domain warm-up is related to but different from IP warm-up. If you send through a shared IP pool at a provider such as Amazon SES, Postmark, or SendHQ, the IPs are already warm and you only warm the domain; if you lease a dedicated IP, you warm both, and AWS says dedicated IP reputation takes around two weeks with some providers and up to six weeks with others.

Which names in a message build reputation?

Every domain a receiver can see in your message builds reputation, so all of them count as 'new' if they have no history. Postmark lists four: the DKIM signing domain (the d= value in the DKIM-Signature header), the Return-Path or envelope-sender domain used for SPF and bounces, the From address domain, and the domains in links, including click-tracking domains. A new subdomain needs warming even when the root domain has years of good history: Customer.io notes that its managed sending subdomains are always new from the inbox providers' perspective. Newly registered domains are the highest risk; Customer.io warns that mail from domains registered within the last 30 days is treated as high risk by Gmail and Microsoft regardless of content or authentication, so register sending domains early and let them age. You also need warm-up after a long pause, after moving to a new provider with different IPs, and after a reputation incident. Before sending the first message, publish SPF, DKIM, and DMARC; warming an unauthenticated domain builds nothing.

How long does it take to warm up a domain?

Warming up a domain usually takes 4 to 8 weeks to reach a few thousand to tens of thousands of messages a day, and the published schedules disagree on pace. Postmark's 30-day plan starts at 50 to 100 emails per mailbox provider on days 1 and 2, doubles while metrics are healthy, then slows to 30 to 50% daily increases and reaches 7,500 to 10,000 per provider by days 22 to 25. Customer.io's plan is more conservative: stages of 2 to 3 days (3 by default) because reputation signals, including Google Postmaster Tools, lag 1 to 2 days behind sending; no stage may exceed 1.5 times the previous one, because doubling is now too aggressive; and it takes about 60 days to go from 100 to 222,000 messages a day. The difference reflects their customers: Postmark serves mostly transactional mail, which has naturally high engagement, while Customer.io serves marketing broadcasts. You only need to warm to your real daily volume; a product sending 3,000 receipts a day is done when it holds 3,000 cleanly.

What does a day-by-day ramp look like?

A domain warm-up schedule is a table of daily volume caps that you hold or increase based on results. The schedule below follows Customer.io's 1.5x-per-stage rule with 3-day stages, which suits marketing and mixed traffic; transactional-only senders with very engaged recipients can move faster, closer to Postmark's doubling in week 1. Volumes are totals across all providers; split them so Gmail, Microsoft, and Yahoo each get a proportional share rather than sending the whole cap to one provider. Advance to the next stage only if bounces stay under 2%, complaints under 0.1%, and there are no 4xx deferrals or blocks from any major provider. If problems appear, hold the stage or drop back by 25 to 30%, as Postmark recommends, and fix the cause before continuing.

Stage  Days    Daily cap   Who to send to
1      1-3        100       opened or clicked in last 14-30 days
2      4-6        150       same
3      7-9        225       same
4      10-12      340       engaged in last 60 days
5      13-15      500
6      16-18      750       engaged in last 90 days
7      19-21    1,100
8      22-24    1,700
9      25-27    2,500
10     28-30    3,800       engaged in last 120 days
11     31-33    5,700
12     34-36    8,500
13     37-39   13,000       stop when you reach your normal volume
Rule: never more than 1.5x the previous stage; hold or cut 25-30% on bad signals

Who should you email first during warm-up?

Email your most engaged recipients first, because their opens, replies, and lack of complaints are what build reputation. Customer.io's guidance is to send only to people who opened or clicked within the last 14 to 30 days during the first three stages, and Postmark recommends widening gradually to 60, 90, and then 120-day engagement windows. For a product team, the naturally warm traffic is transactional: password resets, sign-in codes, receipts, and onboarding messages to people who just signed up are expected and rarely reported, so starting a new domain with transactional mail is the fastest safe ramp. Leave cold segments, old imports, and anyone who has not engaged in 6 months until last, and run list hygiene before the first send by removing hard-bounced, role-based, and invalid addresses. Do not use 'warming services' that exchange synthetic opens and replies between seed inboxes: Customer.io states that inbox providers detect synthetic engagement and give no reputation credit for it, and recommends treating such a domain as cold.

What metrics should you watch while warming up?

Watch four metrics per mailbox provider, daily, before every increase. Hard bounce rate should stay under 2%; Postmark's troubleshooting treats over 3% as a problem to fix before continuing. Spam complaint rate should stay under 0.1%, the level both Postmark and many practitioners use as the working ceiling, and must never approach Google's 0.30% limit for bulk senders. Deferrals: a rise in 4xx responses such as Gmail's 421 rate-limit messages or Microsoft's throttling codes means the receiver wants you to slow down. And engagement: falling opens or clicks at one provider while others hold steady usually means that provider is routing you to spam. Sources: your sending provider's per-message events, Google Postmaster Tools for Gmail spam rate (lagging about a day), Microsoft SNDS for Outlook.com IP data, and DMARC aggregate reports to confirm every message passes authentication. SendHQ also enforces its own per-identity limits: in a rolling 7-day window, one complaint at 0.05% or more throttles a From address and two at 0.1% or more pause it, returning HTTP 423 until metrics recover.

What should you do if warm-up goes wrong?

If warm-up goes wrong, stop increasing volume, find the provider and the stream that caused it, and fix the cause before resuming. High bounce rates mean list quality: remove the source segment, validate addresses, and suppress every hard bounce permanently. Rising complaints mean the wrong audience or unexpected content: cut back to recipients who engaged in the last 30 days and check that unsubscribe works, including one-click List-Unsubscribe for marketing mail. Deferrals at a single provider mean you outpaced its trust: reduce volume to that provider by 25 to 30% and hold for 3 days. Spam placement with clean metrics often means a problem domain inside the message, such as a link shortener or a new tracking domain with no history. In SendHQ, a paused identity receives HTTP 423 before provider submission; there is no manual unpause, and the identity returns to active automatically on its next send once its rolling bounce and complaint rates fall below every threshold, so the right move is to fix the stream rather than retry.

# Check per-identity state and recent outcomes in SendHQ
curl -s https://sendhq.cc/api/v1/deliverability/reputation \
  -H "Authorization: Bearer $SENDHQ_API_KEY"
curl -s https://sendhq.cc/api/v1/deliverability/stats \
  -H "Authorization: Bearer $SENDHQ_API_KEY"   # 30-day delivery aggregates

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منابع اصلی

• How to warm up a domain — Postmark
• Safely warm up your sending domain — Customer.io
• گرم‌کردن نشانی‌های IP اختصاصی — Amazon Web Services
• Email sender guidelines — Google
• Deliverability — SendHQ