Mailbox warm-up

Warmup is a controlled period for building reputation. A new domain has no sending history.¹ It involves four weeks of small, real, engaged sending before higher volume begins.² Let engagement set the pace: replies and spam rescues matter more than send count during warmup.³

Decide the scope

Decide which reputation you are building and what volume the finished setup must carry. This keeps a domain plan from hiding a mailbox problem.

Domain warmup and mailbox warmup cover different scopes of the same job.⁴ Email warmup means gradually sending and receiving mail from a new or inactive mailbox.⁵ Warm every mailbox, including replacement mailboxes, before it sends cold outreach.⁶

Work backward from the target and ask what the ramp must accomplish.⁷ Before you configure anything, answer these questions:

• What daily volume will the finished setup need to carry?
• Which recipient providers will receive the messages?
• Which inboxes sit on each domain?
• Which domains should begin warming first?

If you run multiple domains, stagger their start dates. Inboxes within one domain can start together.⁸ For marketing email, create a separate subdomain and warm it before sending at volume.⁹

Prepare the sending setup

Finish the infrastructure work before reputation becomes part of the test. A ramp cannot tell you much when authentication or DNS is still changing.

SPF, DKIM and DMARC must pass before the first send.¹⁰ Validate the current email domain DNS setup.¹¹ Run an independent check too, because a validation result alone is insufficient.¹²

Keep the sending identity stable while the ramp runs. If the domain, branding or service changes, account for the effect on email before you continue. A domain change can influence email, and the wrong move can send messages to spam.¹³

Map recipients and volume

The first warmup step is to examine who will receive the messages and how much they will receive.¹⁴ Track the amount going to Gmail, Yahoo, AOL, Microsoft and other recipient domains, then segment the list by receiver as much as possible.¹⁵ Keep that split visible in your plan so a delivery change has a clear place to investigate.

Treat the warmup list as a test of sending quality. A poor-quality warmup list undermines the rest of the plan.¹⁶ Before you raise volume, make sure you can explain who will receive the first messages and why they are likely to engage.

Run the ramp

Start with a small daily load and increase it in controlled steps. Start with 5 to 10 plain-text emails per inbox per day, then increase the volume across four weeks.¹⁷ Keep the early messages real and capable of producing genuine engagement. Move up only when the current level produces clean signals, and pause when those signals weaken.

Apply the ramp to each mailbox that will send. A domain can have a good reputation while a newly added mailbox still needs its own sending history. Keep the domain start dates staggered when you warm multiple domains.

Do not jump from warmup volume to the final campaign load in one move. Make each increase observable, then give recipient providers time to react before adding more.

Read the signals and set the timeline

Let provider and recipient behavior set the next step rather than the launch date.

Mailbox providers monitor engagement signals such as opens, clicks and spam complaints.¹⁸ If they do not see positive engagement, or users start marking messages as spam, the domain reputation suffers and inbox delivery becomes harder or impossible.¹⁹ Without sender reputation, providers may slow deliveries, send messages to spam or block them entirely.²⁰

When those signs appear, hold the ramp. Check the audience, message relevance, authentication and recent volume before adding more. Domain warming can take longer when providers do not perceive the messages as wanted by recipients.²¹

Use the four-week ramp as the initial operating plan. Maximum deliverability can take four to eight weeks, depending on targeted volume and engagement.²² Keep the ramp open for that range when the signals need more time, and do not declare the domain ready because the calendar says so.

What not to do

Most failures come from treating setup or volume as the whole job. Use these as stop signs while the ramp is running.

• Do not launch cold outreach from an unwarmed domain, because it is the fastest way to lose the domain.²³
• Do not send too much too fast or to poor-quality data, because mailbox providers form an opinion quickly.²⁴
• Do not change the domain casually, because the wrong move can send messages straight to spam.¹³
• Do not end warmup at setup, because it is the period when you teach mailbox providers about the sender.²⁵