421 resomta-h1p-028516.sys.comcast.net Excessive failed authentication, ESMTP server temporarily not avai

I work at a retirement community where we have over 300 residents that live here and a lot of them use Comcast emails and we are noticing more often than before that the outgoing smtp server stops working for residents. Since we've gotten a lot of complaints recently I have been digging through firewall logs, and all kinds of things trying to figure out what's happening. I found today that trying to telnet to smtp.comcast.net I get this error 421 resomta-h1p-028516.sys.comcast.net resomta-h1p-028516.sys.comcast.net Excessive failed authentication, ESMTP server temporarily not available, please contact support which seems like Comcast is blocking our IP address. We provide internet access to all of our residents through a Comcast Metro Ethernet connection over a community wifi network so everyone has the same public IP address. I need to find someone that can resolve this issue ASAP as this has been an ongoing problem and in the past we would just wait it out and a day later it would start working. But now it seems like every day it's happening and constantly happening way too much that it's becoming a huge problem for our residents. They are all 65+ and have had their Comcast emails since Comcast probably came out with an email back in the day so they do not want to switch from comcast email otherwise I would just recommend them to go with another provider. So since that is the case we need to find someone that is capable of figuring out this issue for us.