Re: [exim] Route to another host if mailbox doesn't exist.

On 02/10/2013 00:06, Steve Wilson wrote:
> I'm trying to plan the migration of a whole domain's email away from
> google over to exim and would appreciate any feedback from the list as
> to the best way to achieve this and what tools are available.

(curiously enough, I just helped a company go the other way with their
user's email.)

> I'm aware of imapsync and have used it frequently to take backups of
> mailboxes before deletion but would like a nicer way, if possible, to
> migrate users with minimal downtime for them.

in terms of mail routing, although you could use the Google 'fallback' mail
routing, and then gradually remove users from Google, that's a bit 'big
bang' for me. I'd go for a different approach that will give you more
control - parallel delivery.

I would create aliases on your 'external' Exim server to route user's emails
to Google (you can use the temporary domain alias that it would have created
- ours was <domain.com>.test-google-a.com or else create another), as well
as to your imap server, i.e.

(in /etc/aliases, or equivalent file)
emailaddr:
username@???,mailbox@???

for all of your users email addresses, with appropriate emailaddr, username,
and mailbox values.

and then switch over your MX to deliver through you, passing on mail to
Google for those users who have not been migrated yet. Gradually, as you
migrate them, remove the entry that forwards a copy to their Google mailbox
leaving just the one that routes it to their IMAP server.

(If your 'external' Exim server and IMAP server are one and the same, then
it's not so straight forward. But I'm sure the idea could be adapted
appropriately.)

Then you just need to deal with backfilling the IMAP folders (there are some
patches available to imapsync, that implement Google's OAUTH mechanism so
that you wouldn't need every individual user's password for Google; and it's
possible with systems like Dovecot to implement a fallback authentication
mechanism using checkpassword that would let you do similar on your own IMAP
server. But that's not really a topic for the Exim users list!)

James.