Meaning of "Discarding" messages from sync_client?

[Jason L Tibbitts III]

>>>>> "WC" == Wesley Craig <wes at umich.edu> writes:

WC> This is the client trying to re-sync the protocol with the
WC> server. There should be a line preceding the "Discarding:" messages
WC> with containing the word "invalid" or "illegal", which is the actual
WC> problem.

Indeed:

Mar 25 23:17:36 util6 sync_client[31645]: MAILBOXES: Invalid unsolicited
  response type 1 from server: 0

Not much idea what that means, though.

WC> At a guess, I'd say that things are messed up.

Joy.  Also, I happened to look in /var/lib/imap/sync and found over four
thousand files with names beginning with "log-" going back years.  I
can't imagine that's good.  A spot check of a few folders shows that the
replica is indeed receiving new messages, but that it's also holding on
to messages which have long since been removed from the master.  I'm not
seeing anything that's being missed entirely, but it's not all that easy
to say for sure.

Is there any reasonable way to get the process back on track, or should
I just sync_reset on the replica and delete the spool?  I guess if I
need to do that, I should just start replicating to the new server.

A related question, perhaps: if I set up a fresh replica, is there
anything I need to do on the master to reset the process (besides
restart the service, of course)?

 - J<