What the 21 days are
The ramp is a schedule, not a promise about the receivers. On day one a new mailbox sends a handful of warming emails; the volume climbs a little each day until it reaches the plan default of 30 a day, and by day 21 the mailbox is at full volume. Nothing about that schedule forces the receivers to agree, which is why the ramp and the placement test are different questions.
Warming does not stop when outreach starts. It drops, because your real sending now carries part of the signal, and the two together should stay inside a volume the receivers read as one person's working day. This is how MailStrike sets it.
| Outreach a day, per mailbox | 1 mailbox | 2 mailboxes | 3 or more |
|---|---|---|---|
| Not sending yet | 30 | 21 | 18 |
| 10 emails | 30 | 21 | 18 |
| 20 emails | 25 | 18 | 15 |
| 30 emails | 20 | 14 | 12 |
| 40 emails | 15 | 11 | 9 |
| 50 emails | 10 | 7 | 6 |
The rule behind the table: a mailbox on its own that is not sending yet warms at the plan default of 30 a day, and once outreach starts it drops one warming email for every two you send, down to a floor of 10. Put a second mailbox on the domain and each one needs 70% of that figure, three or more and each needs 60%, because the domain earns the signal across all of them. Volume is a setting per mailbox, so you can dial these in directly.
None of that touches the tool your campaigns go out through: warming runs in the background beside your existing stack, at a daily volume you set per mailbox.
Inside that schedule the receivers are doing their own arithmetic. Google and Microsoft decide where mail lands from authentication, sending history, content and engagement, and a mailbox that has been warming for three weeks has history where a new one has none.
What sets the timeline
- Domain age and history. A domain under 30 days old carries no trust and has the most to prove. A domain with clean past sending has reputation to build on; one that sat dormant has to show it is active again; one with negative history has to undo it first. The table's age bands exist because these differ.
- Authentication. SPF, DKIM and DMARC have to pass before day one; engagement on a domain the receiver cannot verify counts for less. Run the auth checker first.
- The receiving provider. Google and Microsoft weigh the same signals differently, which is why the table has a row per provider rather than one number.
- Engagement quality. Opens alone are a weak signal. Replies, dwell, mark-as-important and spam rescues from real mailboxes, varying from mailbox to mailbox, accumulate trust faster than uniform warmup traffic.
What speeds it up, what stalls it
You cannot safely compress three weeks into three days: the receivers watch behavior over time, and a sudden volume increase is the exact shape their filters are tuned to. What you can do is make every day of the ramp count.
Accelerators: authentication fixed before day one, the default ramp left alone, sending in your working hours, and engagement that reads as a team of different people. That last one is what an AI Agent on each mailbox produces, because each agent has a persona: its own reply rate, dwell time, active hours and writing style, so several mailboxes on a domain do not share one footprint.
Stalls: a failing DKIM key or an SPF record past the ten-lookup limit, a volume jump, identical behavior from every mailbox, and problems warming cannot fix on its own, such as a blacklist listing (check with the blacklist checker) or a complaint rate above Google's 0.30% line.
When to start outreach
The signal is the dashboard saying the mailbox is safe to send from, plus a placement test that shows the mail landing in the inbox across providers, not the calendar on its own. On a new domain that lines up with day 21. Start at 20 to 30 sends a day per mailbox and grow as placement holds, and keep warming running: after the ramp it settles to about 15 to 25 a day and never stops, so the record that carries the campaigns keeps building.
The short version
Twenty-one days on the default ramp to full volume, improvement visible around two weeks. The measured days to 90% inbox by domain age and provider are in the table. Authentication first, no volume jumps, engagement that varies by mailbox; then outreach at 20 to 30 a day with warming continuing underneath.