Update 11/13/2023: We added new information on how to know when this change is released to your organization.
Today, we are announcing an update to our requirements for SMTP relay through Exchange ...
Updated Feb 22, 2024
Version 19.0
Blog Post
Carolyn_Liu
Microsoft
Microsoft>>>
"However we do have a few systems that send via the relay from a sub-domain of that of the relay, hence the From/Sender domain and the Return-Path are both the same but are sub-domains of the relays e.g. sub.relaydomain.com.
The sub domains are not published in O365 but the parent domain (the same one as that of the relay) is.
Will this use of sub-domains cause any issue with our IP based connector, is the matching strict or relaxed?
"
Answer:
Is your accepted domain an InternalRelay type? If so have you checked MatchSubdomain in the accepted domain config? If so that's why it is working and you will be fine.
We will also publish a new property in MessageTrace in the near future, this property will tell you if the mail is using 1.c to accept the mail. Then you can tell exactly you are impacted or not.