I was successfully using smtp.comcast.net as a smart host on my Exchange 2003/2010 server for years and years. Suddenly it stopped working. My protocol logs showed a successful connection to smtp.comcast.net. My sender and recipient E-Mail addresses were accepted but the mail never arrived at the intended destination. I enabled TLS as is now required but no go. I switched to a free third-party SMTP provider. I just tried with my Comcast SMTP conector a few days ago and lo and behold it works now. I guess xFinity fixed something on their end.
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