Hi @ Brandon Hofmann ,

Great to know that you've already got a solution and really appreciate it for your sharing!

By the way, since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others.". and according to the scenario introduced here: Answering your own questions on Microsoft Q&A , I would make a brief summary of this thread:

【Anonymous Relay Not Working When Sender Address Has Name/Alias After Redoing HCW】

Issue Symptom:

Last week we had to update the certificate for our hybrid connector, somehow during that process we managed to blow out most of our connector settings.

We have most everything working now, except our What's Up Gold service is unable to send alert emails via our Anonymous Relay (from on-prem to Exchange Online) when we have an alias/name in the Sender address (i.e. "ServerName is Down email@domain.com"); if we change the Sender address to just "email@domain.com" it sends successfully, but with the alias/name we get " The exception is SMTP protocol error. 501 5.1.7 Invalid address".

The Solution:

Within the same week of the Exchange cert upgrade, we also updated What's Up Gold, and it turns out they increased their SMTP security, so that is what is causing the issue.

I confirmed this by using the old version of What's Up Gold, that uses the same relay, and it worked properly there.

You could click the "Accept Answer" button for this summary to close this thread, and this can make it easier for other community member's to see the useful information when reading this thread. Thanks!