I too have received complaints from my MSN friend who sends emails to me but gets an "undeliverable" reply from their MSN ISP. I tested by sending an email from my Gmail account to my Comcast account and all seems to be working ok. Wonder why MSN emails are having trouble?
Looks like overnight 3/12 - 3/13 the problem has been corrected. I'm finally getting all the backlogged emails from my MSN account friend. There must have been a glitch in the Comcast mail server blocking MSN emails but all seems to be ok (for now anyway.)