Exchange Online: Updated requirements for SMTP relay as of November 1, 2023

Exchange Logo[German]Quick note for administrators who use Exchange Online as an SMTP relay to send bulk email. Microsoft is adjusting the requirements for using SMTP relay functionality accordingly as of November 1, 2023. The company made the announcement in a June 19, 2023 blog post.


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I came across the topic via the following tweet from Scott Schnoll, senior product marketing manager for Exchange Online, Exchange Server, and Microsoft 365 Networking.

SMTP-Relay requirements for Exchange Online

The details can be found in the Techcommunity post Updated Requirements for SMTP Relay through Exchange Online, which lists the previous two conditions that must be met to use Exchange Online as an SMTP relay. Currently, one of the following conditions must be met as the organization's accepted domain in order to accept and relay email through Exchange Online as an SMTP relay:

  • SMTP certificate domain on the SMTP connection; or
  • SMTP envelope sender domain in the MAIL FROM command (P1 sender domain); or
  • SMTP header sender domain as displayed in e-mail clients (P2 sender domain).

Alternatively, it is sufficient if the IP address of the sending host or the certificate domain on the SMTP connection matches the client's OnPremises type inbound connector. Starting November 1, 2023, the SMTP P2 sender domain matching condition will be removed. Corresponding emails will then no longer be accepted. Emails can then only be sent via SMTP relay if the remaining conditions are met.

If organizations need to route email from on-premises through Exchange Online and some of that email applies to the above scenarios, administrators need to update your OnPremises type inbound connector to use a certificate domain (instead of IP addresses). Administrators must also add the certificate domain as an accepted domain of the organization in question. Details can be found in the Technet article.


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An SMTP relay server, mail relay server or smarthost is a mail server that accepts mail from a sender and forwards it to any third party. An SMTP relay server is used by enterprises to send bulk email through a separate domain and email server.


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