workaround for IMAP server-side timeout during mail splitting

[Kalle Olavi Niemitalo <kon@iki.fi>]

I had a problem where my email provider's IMAP server closed the
connection while Gnus was splitting received mail to nnml
folders, and Gnus was then unable to expunge those messages from
the server.  I worked around it by customizing the mail-sources
variable and adding various date limits to the predicate.
I hope this information will help someone else.

Background:

I use Gnus to download my email over IMAP and then read it on my
computer.  I don't normally keep any email on my email provider's
server.  Recently though, I neglected my computer for several
weeks, and the email accumulated on the server.  When I resumed
using the computer, Gnus first downloaded the email, then split
it to folders, and finally tried to expunge it from the server;
but the server had apparently timed out and closed the connection
because the splitting took so long.  I retried fetching the mail
but got the same error again.  I monitored the IMAP responses
using strace and saw that retrying the operation made Gnus fetch
the UID numbers from the IMAP server again.

Workaround:

I customized the mail-sources variable and added a date limit to
the predicate, like this:

UNSEEN UNDELETED BEFORE 22-Mar-2018

That let Gnus download a smaller number of messages at a time,
and finish splitting them before the server timed out.  I then
kept incrementing the date in the predicate until I got all the
messages and could remove the date limit.