I'm having the same problem. Server hosted in AWS. Domains are @oaktreepeak.com and @silverspore.com. Hasd worked for years and reverse DNS has been correct for years as well.
Attempting to go to http://postmaster.comcast.net/block-removal-request.html results in a "504 Gateway Timeout" error.
So I can't send to anyone on Comcast, including much of my family.
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