Support incremental backup for mail content
Nowadays, many users are using IMAP, maybe because is the default setting in many popular email clients (Mozilla Thunderbird, and I remember Mail on Windows 8.1 didn't support POP3 at all).
Mail content usually grow but previously stored messages doesn't change often. In Maildir format, used by Plesk, qmail and Postfix, when an user got new mails, new files with unique name are added on disk, and only some files are changed (basically, only courier-imap and dovecot caches and indexes, perhaps easy to rebuild from scratch).
So, please, support incremental backup for mail content too
We’re already planning to implement this functionality in upcoming releases.