I would like to backup a remote dovecot account to my local disc.
A "dovecot account" is an IMAP/POP3 mailbox. So you'll need to pull down all the mail using an appropriate IMAP/POP3 tool. Never fear! Dovecot has such tools, as you've found out.
The docs explain a tool which seems to support only a "push", not a
"pull" way of backup: http://wiki2.dovecot.org/Tools/Doveadm/Sync
That's just the "sync" part of the utility which merges mailboxes and their items without any loss. You can also use the
doveadm-backup tool (Which is just
doveadm backup which pulls items from one mailbox into another, and deletes conflicts in the destination.
My question: Why not backup a remote dovecot IMAP account via rsync?
Because you're now dealing with two separate layers. Dovecot is an IMAP server and to backup an IMAP account you'll probably be looking to appropriate IMAP utilities.
rsync is a filesystem based tool. Can you use rsync to backup your IMAP accounts? Sure! But you'd be backing up filesystem level information about the IMAP application and its data. Make sure you know where to look and how to recover the data appropriately.
Which rsync options should I use?
The ones that work.