Emails from Comcast never reach O365 (or any other domain) with no trace or NDR

I've actually already resolved this issue, but couldn't find any trace on the internet of anyone else having this problem, so I decided to post my experience.

One of my clients was reporting that Comcast.net users emails were not getting through to them. All traces run in Office 365 showed no attempts made by the Comcast.net user or server at all, thus it not leaving Comcast’s email servers.

Using my personal Comcast.net email account and sending lots of tests to my O365 and gmail accounts, I was able to deduce that their domain (123domain.com) as a word in the body of an email from a Comcast.net account was being dropped at Comcast’s email servers and no NDR was being provided to the Comcast user. To clarify, if I sent an email from my Comcast account TO a user at my client, it would go through. It was only when the domain itself was in the body of the email that it was being dropped. In most cases the domain was being automatically inserted into replies when a Comcast user hit Reply to an email from us, as the domain was being listed after the users real name in the FROM field. eg: FROM: User Name [email protected]

I contacted Comcast a couple times before figuring this out and they were unable to figure out exactly why this was happening. It was only after I figured out exactly what it was myself that they were able to resolve the issue.

Using this information I contacted Comcast’s Internet Security Team at: 888-565-4329. After explain the situation to a few people, one of them finally understood what I was explaining, and told me he found the domain added as a keyword in their spam block list. They use Cloudmark’s spam block list and were able to remove the keyword (123domain.com) and it resolved our issue completely.

He was unable to provide an exact answer as to how our domain got added as a keyword in the spam filtering list, but I am making an assumption that if enough Comcast.net users report an email as spam in the Comcast email portal, then somehow something gets added to the block list automatically. My client doesn't send out spam, but I can see how some of their emails would come off as spam to someone that was not expecting it. Their domain wasn't listed on any blacklists.