Unable to connect to smtp.comcast.net after ISP change

I am unable to use my Comcast account at my work site, as are other users, including the President/CEO.  It will sometimes sporadically work, but 9 times out of 10 it fails with various errors, such as "excessive authentication attempts" or "The server responded: 421 resomta-a2p-647654.sy". 

This seems to be related to a recent change in ISP at that location.  Previously, we were using Comcast Business, and never had any problems.  Recently we switched to AT&T fiber, and have a static IP block of  [Edited: "Personal Information"].

Up until yesterday, the router at work site was assigned IP [Edited: "Personal Information"].  No matter what I tried, connections to smtp.comcast.net were rejected.

So I changed the router IP to [Edited: "Personal Information"].  IMMEDIATELY connections to smtp.comcast.net started working.

Today, connection attempts to smtp.comcast.net are failing with error "The server responded: 421 resomta-a2p-647654.sy"

As an additional test, this morning I changed the router IP back to [Edited: "Personal Information"].  Connections to smtp.comcast.net are working fine again.

Since this problem appears to be related to the originating IP, would it be possible to permanently whitelist the IP block [Edited: "Personal Information"]?

If there is some other reason that connections from this IP block are failing that I can remediate on my end, please let me know what steps I should take.