What is different about warming Microsoft 365
Two things. Microsoft's filters give more weight to the sender's standing than Gmail's do, so a new tenant starts from a lower baseline and a quiet spell costs more. And Microsoft publishes a set of limits that are easy to trip in a way Google's are not: a per-minute throttle, a tenant-wide external recipient limit that depends on your license count, and hard caps on trial tenants and onmicrosoft.com domains. None of them is where a warming ramp stops, but the tenant-level ones decide whether the tenant is fit to warm at all.
Connect the mailbox: the Microsoft OAuth flow
- 1
Authorize with Microsoft
Pick the account on Microsoft's own consent screen (login.microsoftonline.com) and grant access. MailStrike's Microsoft OAuth verification is complete; the password is never shared and the grant is revocable from the account.
- 2
Authentication checked on connect
SPF, DKIM, DMARC and MX are read as the connection lands. On Microsoft 365 the common gap is DKIM not enabled for the custom domain (the selector1 and selector2 CNAME records), and the check returns the records to publish.
- 3
Pick the persona and the volume
A dedicated AI persona per mailbox and a daily volume with the plan default of 30. Warming mail is filed to a warming folder on arrival and never reaches the real inbox.
- 4
Warming starts
Threads with the MailStrike network in your working hours, with every open, reply, rescue and mark-important logged, and Flashman's daily summary per mailbox.
Microsoft's limits, and the ones that matter
Microsoft's Exchange Online limits page documents a recipient rate limit of 10,000 recipients per mailbox per 24 hours, up to 1,000 recipients per message, and a throttle of 30 messages a minute. The tenant outbound limit (TERRL) caps external recipients tenant-wide at 500 × licenses^0.7 + 9,500 per day for paid tenants, 5,000 for trial tenants, and 100 for onmicrosoft.com domains; once hit, mail bounces with 5.7.233 and 5.7.234-class errors.
- The throttle is not a cap. 30 a minute delays mail; it does not reject it. A warming ramp of 30 a day per mailbox is nowhere near it.
- The tenant limit is about the tenant, not the mailbox. A small paid tenant has a daily external ceiling in the low ten-thousands; a trial tenant has 5,000; an onmicrosoft.com sender has 100. Warm on a paid tenant with a custom domain.
- New tenants are restricted. Microsoft applies tighter limits to tenants with no history, and receivers apply tighter filters to domains with none. Both loosen with time and evidence, which is what the ramp supplies.
The full record, with what happens when each limit is exceeded, is on the Microsoft 365 sending limits page.
The 21-day schedule, and how much warming to keep
The default ramp is the same shape as for Google Workspace: low on day one, a steady climb, maintenance after day 21. How much warming to keep running once you start sending is below.
| 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.
SNDS and JMRP: the Microsoft side of the ledger
Microsoft shows senders what it thinks of them through Smart Network Data Services: complaint rates, trap hits and filter results per sending IP, for IPs you register. The Junk Mail Reporting Program forwards individual complaints. For a Microsoft 365 mailbox the sending IPs are Microsoft's, so SNDS is most useful when you also send from your own infrastructure; for the domain itself, the signal you can act on is the placement test.
The 2025 high-volume rules
Since 5 May 2025 Microsoft rejects mail from senders of 5,000 or more messages a day to consumer Outlook.com, Hotmail and Live addresses whose domain does not pass SPF, DKIM and an aligned DMARC record (p=none is the minimum), with 550 5.7.515 Access denied, sending domain does not meet the required authentication level. A domain failing DMARC is rejected with 5.7.509. A cold-email tenant is far below 5,000 a day to consumer addresses, but the authentication floor is the same one placement rests on, and the auth checker tests a domain against all three providers' rules.
After the ramp
That call belongs to the dashboard, not the calendar. Start at 20 to 30 a day per mailbox, scale as Flashman recommends, and keep warming at maintenance volume: Microsoft's standing-based filtering is the one that punishes a quiet mailbox hardest. Your sequencer sends from the same mailbox over its own connection; see the per-tool pages for what each tool's own warmup covers.
Questions about Microsoft 365 warmup
How long does it take to warm up a Microsoft 365 mailbox?
MailStrike's default ramp reaches full volume in 21 days for Microsoft 365 as for Google Workspace. Microsoft's filters weigh the sender's standing more heavily than Gmail does, so a new tenant tends to start from a lower baseline; whether that changes the days to target is what the ramp study on this page will show once it is published.
What are the Microsoft 365 sending limits?
Per Microsoft's Exchange Online limits page: 10,000 recipients per mailbox per 24 hours, up to 1,000 recipients per message, and a throttle of 30 messages a minute. On top of that sits the tenant-wide External Recipient Rate Limit, which for a paid tenant is 500 times the number of licenses to the power 0.7, plus 9,500, per 24 hours; trial tenants are capped at 5,000 external recipients a day and onmicrosoft.com domains at 100.
What is the 30 messages per minute limit?
Exchange Online throttles a mailbox that sends faster than 30 messages a minute; the mail is delayed, not rejected. Warming never approaches it, and a sequencer that respects it will not either.
What does 550 5.7.515 mean?
Since 5 May 2025 Microsoft rejects mail from high-volume senders (5,000 or more messages a day to consumer Outlook.com, Hotmail and Live addresses) whose domain fails SPF, DKIM or DMARC, with '550 5.7.515 Access denied, sending domain does not meet the required authentication level'. Before that date such mail was routed to Junk. Fix authentication first; warming cannot help a domain that is being rejected at the door.
Does MailStrike need my Microsoft password?
No. Microsoft 365 connects over OAuth on Microsoft's own consent screen; MailStrike never sees the password and the grant is revocable from the Microsoft account. MailStrike's Microsoft OAuth verification is complete.
Can I warm up an onmicrosoft.com address?
You can connect it, but Microsoft caps onmicrosoft.com domains at 100 external recipients a day and receivers treat the default domain as a sign of a throwaway tenant. Add a custom domain, authenticate it, and warm that.