Assuming that you haven't made any changes to a working config setup,
the issue is something that's going on at the server. About the only
thing you can do is check with Comcast Support, to make sure they know
about the issue. Beyond that, wait, and things will eventually come back.
There's any number of things that could be going on inside Comcast's
infrastructure, whether a POP server that needs a restart (either the
service itself or the entire server), a problem router, and plenty more
that I can only speculate about.
Before you talk to Support, you should also determine whether you can
send outbound mail from TB or not, and whether or not you can access
your mailbox via Comcast's web client. Besides establishing for
yourself what you're able to do, it could also help Comcast in problem
reporting. In particular, a web client is essentially IMAP, and if you
can get there, it means that web access to that server is OK, as well as
IMAP access (and implication that it's just POP that would be the problem).
As for the error message itself, it's probably not going to be
especially useful except to Comcast Support personnel.
Smith