The Microsoft 365 playbook

Outlook sends your mail. Your domain still has to earn the inbox.

Exchange Online sends through Microsoft’s shared outbound pool, on IP addresses that change often, so the IP is never yours. The domain is, and Microsoft offers no warmup for it. Keep sending from Outlook exactly as you do. Add the domain here and we build its reputation.

Start warming my domainSee Inbox Boost
The Microsoft 365 playbook

Keep sending from Microsoft 365. Warm the domain here.

Step 1

Turn on DKIM for your custom domain

Microsoft signs mail from your onmicrosoft.com domain automatically, but not from a custom domain until you enable it. Publish the selector1 and selector2 CNAME records shown in the Defender portal, keep one SPF record with include:spf.protection.outlook.com, then add DMARC, starting at p=none.

Step 2

Know what Microsoft does, and what it does not

Exchange Online has no warm-up feature. Microsoft says it isn’t suited to bulk mailing and points senders of bulk commercial email to third-party providers, and it caps sending with per-mailbox and tenant-wide limits. From May 2025, Outlook.com requires SPF, DKIM and DMARC from high-volume senders.

Step 3

Add your sending domain to Inbox Boost

Add the domain your mailboxes send as, the one in your From address, such as yourcompany.com, and we set up what it needs and build its sender reputation for you. Nothing to connect to Microsoft 365: no admin consent, no app, no change to your tenant.

Step 4

Keep everyday mail and outreach going from Outlook

Replies, proposals, sales email and the tools that send through your mailboxes: all of it stays as it is. Mailbox providers score the domain in your From address, so the reputation built here counts for every mailbox sending as yourcompany.com.

Step 5

Check where your mail lands

Inbox placement tests show whether your mail reaches Primary, Promotions or Spam across the major mailbox providers, and blacklist monitoring keeps watching the domain once it is warm.

  • Nothing to connect: no admin consent, app or add-in
  • Nothing to change: mailboxes, users and mail flow stay exactly as they are
  • Every mailbox on the domain: the reputation belongs to the domain, so they all share it
  • Watched after warmup: blacklist and authentication monitoring on your sending domain
Features Microsoft 365 senders lean on

The parts of the platform this plan leans on.

Feature

Inbox Boost

Add a domain. We provision the sending server and publish your DNS.

Learn more Feature

Guided DNS setup

We write SPF, DKIM and return-path into your zone, not you.

Learn more Feature

Inbox placement testing

See whether you land in Primary, Promotions or Spam across providers.

Learn more
See it play out

A Microsoft 365 domain, warmed without pausing a single send.

Four checkpoints, in order. Click any step or let it play.

microsoft 365 · yourcompany.com
DKIM (selector1, selector2)enabled
DMARCpublished
domain reputationnot built yet
Questions

Microsoft 365 questions, answered.

Does Inbox Boost need access to our Microsoft 365 tenant?
No. There is no admin consent, app or integration to install. Inbox Boost works on the sending domain, and your mailboxes keep sending from Outlook exactly as they do now.
Microsoft sends from its own servers. Why does my domain matter?
Exchange Online mail leaves through Microsoft’s shared pool, on IP addresses Microsoft says change often, so the IP is never yours. The domain is the reputation you can build or damage, and that is what we build.
Microsoft says Exchange Online is not for bulk mail. What does that mean for us?
Use it for the mail it is built for: people’s own mail and sales email. Microsoft points bulk commercial email to third-party providers, and recommends sending mail from services you do not directly control from a subdomain, so their problems do not affect the main domain.
What do the Outlook.com sender requirements mean for us?
From May 2025, Outlook.com, Hotmail and Live addresses expect high-volume senders to pass SPF, DKIM and DMARC, with DMARC at least at p=none and aligned. Non-compliant mail goes to Junk. Enable DKIM for your custom domain first, then build its reputation here.
Can I see whether it is working?
Yes. Inbox placement tests show where your mail lands, Primary, Promotions or Spam, across the major mailbox providers, and blacklist monitoring keeps watching the domain afterwards.
About the Microsoft 365 facts on this page

Checked against Microsoft 365's own documentation on 12 October 2026. Microsoft 365 is a trademark of its owner; SenderSignal is not affiliated with it.

Other mailbox providers: Google Workspace. All use cases.

Outlook keeps sending

Warm the domain behind your Microsoft 365 mailboxes.

Start free, add the domain, and keep every mailbox sending from Outlook while its reputation builds.

Start freeHow Inbox Boost works