We will be deploying a replacement Dovecot server soon, and we are planning to use maildir for the primary storage, but with an archive namespace using mdbox (or perhaps sdbox), and SIS.

Our backup servers and (luke)warm spare server need to obtain full copies of the mail store. For the maildirs I know I can simply use rsync (we already use it here).

I'm a little wary of using rsync with mdbox and SIS though.

Significantly limited knowledge opinions below:

Probably not the answer you want - but I would strongly suggest using Dovecot replication.  Dovecot replication Just Works - so don't reinvent the wheel when Timo provided such a polished tool already.  And based on my previous SIS experience - while dbox is nice I would suggest avoiding SIS until there are reports of more development.  Sdbox will be solid - which is what you want for an archive - though maildir would be the safest.  Archives don't need to be rapid-access - they need to be dependable.  SIS is wonderful for space saving - but until there's more safety checks built-in I'd suggest avoiding it for production backups.  Drives are cheap - lost data, lost time, lost hair, lost sanity...is not.

I'm aware of "doveadm backup", and (although it currently throws up a few errors) it seems like it might be a valid solution for our warm spare server. Our backup servers, on the other hand, aren't supposed to be visible to the production machines, with the exception of the backup machine sshing in to do an rsync each night. We can't install dovecot on them.

The backups don't have to be "visible" to other machines - don't even have to be running IMAP/POP services (I think).  And the replication command is run - via ssh (see https://wiki.dovecot.org/Replication) - so what's the problem?

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Daniel