Hi,

I can only write about my experience:

2. On the new Debian host, is there a better filesystem than EXT4 that I should use for the mail storage? I have used ext4 and never had problems with it for ages. Since recent times I also use ZFS on linux, but it's a bit slower and eats lots of RAM as you probably know

3. On the new host, is mdbox generally recommended over maildir? Especially when mixed with dsync? Generally yes, brings speed a little speed boost, however administration is not as simple as with maildir (no systemtools like cp, rsync etc; you have to use dovecots tools); don't know in combination with dsync since I have not used it except for backup.

4. Does dsync work as a good backup tool to keep a mirror of my live system? yes, so far I have not had any troubles

5. I don't know Linux as well as FreeBSD but is it recommend to setup LVM and take daily snapshots as part of the backup plan? Do LVM snapshots cause any problems with Dovecot in the case of a rollback/restore? Generally, LVM and snapshots are nice. However, We had some troubles with it: having snapshots will have a significant impact on your performance; we have also encountered problems with rollbacks where removing snapshots got stuck; this was one of the reasons why we switched to ZFS (or: we are in process of switching)

and just for info: my systems are all quite small (<1000 users).

regards, infoomatic