Can't send emails to external domains in a Group

Dear Support!

We have a few Groups in the AD, and in the group members there a few email addresses, that are point to an external domain. These messages are not delivered:

I am deeply sorry, but I was not able to deliver your mail to the following
addresses: : #5.1.0 Address rejected.
This is a permanent delivery failure. The delivery has been tried 0 times
and will not be rescheduled again.

What am I missing?
Thank you!


Can you share the Axigen log (everything.txt by default) for such message.