emails to comcast users getting bounced...

One of my clients sends mailing lists out (maybe 1-2x a week) and there are less than 10 comcast users on the list.
They are starting to see regular bouncebacks with:
550 5.4.300 Message expired -> 451 4.2.0 Throttled - https://postmaster.comcast.net/smtp-error-codes.php#RL000010‎

and

550 5.4.318 Message expired, connection reset (SuspiciousRemoteServerError), 450 4.4.318 Connection was closed abruptly (SuspiciousRemoteServerError)‎

the domain sending the messages is hosted by MS so should be reputable...

with less than 10 comcast users (but not less than 10 total) i wouldn't think it would be a "mailing list" issue...but i thought i'd reach out for any ideas.

thanks