[Dovecot] POP server migration and duplicate messages.

[Ernie Dunbar]

I'm having trouble with Dovecot where my mail client (if it matters, I'm
using Evolution) collects some duplicate messages when switching between
POP servers that have the same messages on them. I'm doing this as a test
for server migration for all of our clients.

Here's my procedure:

1. set up Dovecot and Exim on the new server so that everything works like
on the old server. The old server also uses Dovecot and Exim, so very few
things changed.
2. On the new server, use NFS to access the users' Maildirs on a remote
NFS server.
3. Sync the Maildirs on the NFS store from the old server with SCP (and I
do a quick SCP sync on my own Maildir during testing to ensure that this
process is quick).
4. Check mail in Evolution on the old server.
5. Change Evolution's server settings to use the new server.
6. Check mail again.

Evolution is set to keep messages on the server for one week, and about 20
of the 184 messages on the server keep getting re-downloaded. From a
random sampling, the UIDL of the messages checks out between both servers,
so I really don't understand what's going on here. If this is just a bug
in Evolution, then that means I can just go ahead with the full migration
and hope that people aren't going to be stuck with re-downloading their
entire mailbox which in some cases, means hundreds or thousands of
messages.