Send password resets and other account-critical messages from a dedicated transactional subdomain, and send marketing or outreach from a separate subdomain. Microsoft’s guidance recommends a subdomain for bulk services outside your direct control so that problems with those services do not affect mail sent from your main domain. Separation reduces that exposure. It does not guarantee inbox placement, and each subdomain still needs its own SPF, DKIM, and DMARC configuration.
Why password resets and outreach should not share a sending identity
A password reset is the message a user is waiting for, often at the moment they are locked out. Marketing and outreach mail, by contrast, comes in larger volumes, is sent to recipients who may not know the sender, and is more likely to generate complaints, bounces, and unsubscribes. When both streams send from the same domain, a weak campaign can damage the reputation that the reset message depends on.
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Microsoft’s DMARC guidance states the risk plainly: “You don’t want issues with mail sent from those email services to affect the reputation of mail sent by users in your main email domain.” (Microsoft Learn, Set up DMARC to validate email in Microsoft 365.) Its outbound-spam guidance is more direct: “Consider using a custom subdomain exclusively for bulk email.” (Microsoft Learn, Outbound spam protection.)
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The reasoning is containment. Mailbox filters weigh several signals, including authentication, sender reputation, content, and how recipients interact with past messages. Keeping bulk traffic on its own subdomain limits how much of that reputation history a bad campaign can touch. It is a risk-reduction step, not a promise that resets will always arrive.
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Set up separate sending streams
Use one subdomain for each job. The labels are examples, not required names. Microsoft’s own examples use t.contoso.com for transactional mail and m.contoso.com for marketing mail. The table below shows a common layout for a site that owns example.com.
| Stream | Example subdomain | Typical messages | Sending rule |
|---|---|---|---|
| Transactional and account-critical | security.example.com or t.example.com |
Password resets, sign-in alerts, verification codes, order receipts | Send only messages the user triggered; keep this stream free of promotional content |
| Marketing and outreach | mail.example.com or m.example.com |
Newsletters, campaigns, cold outreach, promotions | Use the provider’s bulk-sending tools; monitor complaints and bounces closely |
Before you change anything, confirm that your sending platform can send from and authenticate each chosen subdomain. A stream that the provider cannot sign for cannot pass DMARC.
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Authentication for each subdomain
Separating streams only works if every subdomain is authenticated in its own right. SPF, DKIM, and DMARC each do a different job.
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SPF: list the authorized senders for each subdomain
SPF publishes the servers allowed to send mail for a domain. Each subdomain needs its own SPF record. Do not assume that a subdomain inherits the parent domain’s SPF record. Keep exactly one SPF record per domain or subdomain, because multiple records can cause a permanent SPF error.
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SPF also has a lookup budget. Microsoft’s current SPF guidance (retrieved 2026) describes a limit of 10 DNS lookups during SPF evaluation. Each included service adds lookups, which is one reason to give bulk services their own subdomain rather than stacking them onto the primary domain’s record.
DKIM: sign with a domain that matches the visible sender
DKIM attaches a cryptographic signature to each message, and the signing domain is published in DNS. For a DMARC pass, the signing domain must align with the visible From domain. Configure the sending service to sign with each subdomain, and publish the key records it gives you. Microsoft’s DKIM guidance recommends subdomains for services outside your direct control for this reason.
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DMARC: decide what receivers do with failures
DMARC tells receiving servers how to treat messages that fail aligned SPF or DKIM checks, and it can return aggregate reports to you. DMARC passes when at least one of SPF or DKIM passes and aligns with the visible From domain. A DMARC record at the parent domain can apply to subdomains unless a subdomain publishes its own DMARC record. Review those reports, because they show which services send on your behalf and whether they are authenticated.
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- Choose one subdomain for transactional mail and one for bulk mail, and confirm that your sending provider supports both.
- Confirm who controls DNS for the parent domain, so the records can be published and changed without delay.
- Publish one SPF record for each subdomain, and count the DNS lookups it triggers against the 10-lookup limit.
- Enable DKIM signing for each subdomain in the sending service, and publish the DKIM key records it provides.
- Publish a DMARC record and review its aggregate reports for unexpected senders.
- Send test messages to a few mailboxes, and check the authentication results in the message headers for SPF, DKIM, and DMARC passes.
- Recheck the records whenever you add or change a sending vendor.
Architecture options compared
There are two plausible designs: separate subdomains under one organizational domain, or separate registered domains for different streams. Microsoft’s guidance directly supports the subdomain approach. It does not provide a direct comparison with separate registered domains, so the table marks those cells as not stated rather than assuming an answer.
| Comparison point | Separate subdomains (one organizational domain) | Separate registered domains |
|---|---|---|
| Reputation-risk boundary | Microsoft recommends subdomains for bulk services outside direct control, to limit effects on the primary domain’s mail | Not stated in the reviewed Microsoft guidance |
| Authentication setup | Each subdomain needs its own SPF record and DKIM signing, aligned for DMARC | Not stated in the reviewed Microsoft guidance; each domain would need the same SPF, DKIM, and DMARC work |
| DNS and SPF complexity | Records are managed under one parent domain, but each subdomain still has its own SPF lookup budget | Not stated in the reviewed Microsoft guidance |
| Cost and ownership | Requires no additional registration in the cases Microsoft describes | Requires an additional registration; cost not stated in the reviewed Microsoft guidance |
Do not treat separate registered domains as automatically better. The evidence supports subdomain separation as a sensible default for most organizations.
Limits and cautions
- Subdomain separation is not a guarantee of inbox placement. Filtering also considers content, sending behavior, and recipient interaction.
- The available Microsoft guidance states its rationale and recommendations. It does not establish how every mailbox provider handles subdomain reputation, and it does not quantify any deliverability improvement.
- Microsoft’s outbound-spam guidance states that sending bulk email through Microsoft 365 is best-effort and is not a supported primary bulk-mail use case. If you use Microsoft 365 for account mail, send marketing through a dedicated bulk-sending provider, and check current Microsoft service terms for your plan.
- Keep account-critical messages authenticated and monitored after you separate the streams. A new subdomain with no DKIM or DMARC configuration gives no protection.
- Vendor features change. Verify each provider’s current subdomain, SPF, and DKIM documentation before you rely on it.
Sources
- Microsoft Learn, Set up DMARC to validate email in Microsoft 365: subdomain guidance, DMARC alignment, and inheritance rules.
- Microsoft Learn, Outbound spam protection: custom subdomains for bulk email, the marketing and transactional examples, and Microsoft 365 bulk-mail limitations.
- Microsoft Learn, Set up SPF to identify valid email sources for your Microsoft 365 domain (retrieved 2026): one SPF record per domain or subdomain, and the 10-lookup limit.
- Microsoft Learn, How to use DKIM for email in your custom domain: subdomains for services outside direct control, and DKIM alignment.
- Microsoft Learn, Best practices for sender authentication support: the roles of SPF, DKIM, and DMARC.
- Microsoft Learn, Best practices for email marketing: reputation, filtering, content, and recipient interaction.
- Microsoft Learn, Mail flow best practices for Exchange Online, Microsoft 365, and Office 365: SPF, DKIM, and DMARC overview.
Microsoft’s examples are vendor guidance rather than statements by an individual, and this article does not rely on independent deliverability testing or measured results.
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