RE: Dynamics GP email with SMTP configuration
Normally, changing the domain wouldn't cause EXCHANGE/SMTP emailing to stop working, unless the Exchange server is on-prem and those users that had access to the Exchange server previously no longer do as the domain changed.
You mentioned that when the users enter their credentials when prompted by the Exchange Log On window, it states they are invalid.
I'd recommend starting with this email troubleshooting guide, as that is where we have pretty much all of our steps/requirements for both MAPI and Exchange emailing:
community.dynamics.com/.../dynamics-gp-email-troubleshooting-guide-1743830067
Usually with the user's credentials not making it through the Exchange Log On window, the most common causes, as mentioned in this guide, are:
--Multi-factor authentication is enabled on the user account, which causes issues unless you're on GP 18.3 and using the new Azure MFA functionality.
--'basic' authentication is disabled on the Exchange environment
--Autodiscover is not working
Again, with the domain change, the domain user credentials that may have been valid to login to Exchange previously may not work now, and your Exchange admin will need to verify that.
This guide and the information above is more a troubleshooting guide from the Dynamics GP functionality perspective.
Thanks