Impact of blocking direct delivery to @domain.mail.onmicrosoft.com addresses in a hybrid environment

Question

Dear Experts,

Could you please advice if there is any impact blocking all incoming external emails address to mailboxes with smtp address xxxx@keyman .mail.onmicrosoft.com ?

We have noticed those emails are not passing via our spam filter and this is critical security issue.

Also, could you please advice what is the best practice in this kind of scenario?

Many thanks!

Answer 1

You can do that, yes. There will be no impact as long your hybrid connector is correctly setup and you only accept messages for that domain from on-prem servers that you controls.

ALso see:
https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/manage-mail-flow-using-third-party-cloud