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
This feature was implemented a very long time ago, but we missed updating the feature request entry accordingly. Please excuse the delay. The reason that it was missed was probably that back then a huge number of users were still using the mbox mail storage format. There it was not possible to get a delta of mails.
In Plesk Obsidian an incremental backup includes mail data changed since the time of the last backup (full and incremental).