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>I would like to understand in what scenarios an email can be sent from a sub domain that is not an accepted domain in Exchange online? (Because if i add a domain in Microsoft Admin center its automatically added in accepted domains.)
Answer: You have to either satisfy 1.a or 1.b in the blog. If your intention is to relay emails from subdomain.mycompany.com, and only mycompany.comis accepted domain but subdomain.mycompany.com is not. then the only way you can relay is satisfy condition 1.a., ie. create Inbound connector (Your Org->Office 365) and uses a cert domain, this cert domain is mycompany.com (or any other accepted domain in your org.)
May I also ask why is it confusing? There must be something does not click, I just don;t know what it is and would like to know.