When using Gnus with an IMAP mail-source, it goes through, reads one
message at a time, sorts them away, and then tries to do a bulk delete
of all of the messages it's read.  This is all fine and within the
bounds of the IMAP spec; unfortunately, not all software is.  I
somewhat regularly wind up using an imtest binary with a 500-byte line
limit, and if I get around that, the local server has a 16K line
limit, which I do run into with enough mail.

It'd be nice if there was a switch or fallback mode or something such
that Gnus didn't always lose in exciting ways when it hit this.  The
"I'm away for a week so it's sure to die, slow and correct is better
than fast and broken" patch is: