Is there a recent change to comcast SMTP service?

I'm the moderator of an academic mailing list.  I've written a program to distributed approved emails to the appropriate listserv.  Since I have comcast for internet, I send them out via smtp.comcast.net port 587 using TLS.  This has worked without problem for the last number of years.  However, starting last week (it might have been a little earlier than that, but the mailing list has fairly low volume) emails that I send seem to fall into a black hole.  I did some debugging, and the dialog from the comcast SMTP server indicates that the email was accepted.  I even tried to have it (as a test) sent to my work id instead of the listserv.  It never appeared.  So is comcast blocking such?