How to Warm Up a New Sending Domain (and What to Check First)

Why a new domain starts at a disadvantage

Gmail, Outlook and Yahoo decide where your mail lands partly on what your domain has sent before: how much, to whom, and how those people reacted. A new domain has no record, so a sudden burst of mail from it looks like the pattern spammers produce. Google says so directly in its sender guidelines: start with a low volume to engaged users, increase it slowly, and avoid sudden spikes if you have no history of sending large volumes.

Warming up is just building that history on purpose, at a pace where every week of good results earns the next increase.

Before day one: four checks

A warm-up only works if the mail you send during it is mail you would be happy to be judged on. Fix these first, because a problem found in week two has already cost you week one.

1. Authentication is published. SPF and DKIM for the sending domain, and a DMARC record. Google requires all three from anyone sending 5,000 or more messages a day to Gmail, and the domain in your From header has to align with SPF or DKIM. Run the domain through the deliverability checker to see each record and what is wrong with it.
2. Authentication actually passes. Published is not the same as passing. Send yourself a test, copy its raw headers, and paste them into the email header analyzer. You want spf=pass, dkim=pass and dmarc=pass in the Authentication-Results header.
3. The content would pass a spam filter. An unsubscribe link on anything marketing, a postal address, no hidden text. The spam checker flags these before a filter does.
4. The email renders. The first sends go to your most engaged recipients, the people a warm-up depends on. A layout that collapses in Outlook or turns unreadable in dark mode is a bad first impression to give them. Check it with the CSS compatibility checker.

A ramp you can follow

Google gives no numbers. Resend publishes a starting schedule for a brand-new domain, which is a reasonable default for any provider:

Day   Messages per day
1     150
2     250
3     400
4     700
5     1,000
6     1,500
7     2,000

From day 4 it also caps sending per hour, starting at 50, so the day's volume is spread out rather than sent in one burst. For targets past the first week, Resend's warm-up calculator extends the ramp from your target volume and start date. A domain that already has a sending history starts much higher, at around 1,000 messages on day 1.

Send the early volume to the people most likely to open: recent sign-ups, active customers, anyone who has clicked lately. Leave old or cold segments for after the domain has a track record, or leave them out.

The numbers that tell you to slow down

• Spam rate. Google asks senders to stay below 0.10% in Postmaster Tools and never reach 0.30%. Resend's warm-up guide uses a tighter 0.08%.
• Bounce rate. Resend's threshold is 4%. A rise usually means an old or bought list, and every hard bounce is evidence against you.

If either climbs, hold the volume where it is, find the cause, and resume once it is fixed. Pushing through a rising spam rate is how a warm-up turns into a recovery.

Decide your subdomains first

If you are going to split transactional and marketing mail across subdomains, decide that before warming anything. Warming one domain and then moving the mail to another throws the work away. The case for splitting, and a DNS layout for it, is in transactional vs marketing subdomains. Set the DMARC policy for them with the DMARC record generator.