I plan to backup Outlook emails from desktop computers to NAS. The tool of choice is rsnapshot as I can install it on the old NAS (Synology DS 1010+). Rsnapshot uses rsync for its job. The classic Outlook (2016) saves messages in pst or ost files, which can grow to gigabytes when contain many email messages. The --append rsync option can considerably speed up the backup process of large files. I am concerned if it is safe to backup pst and ost files using this option. Namely, after receiving or sending a new email Outlook probably appends these emails to corresponding pst or ost files. However, when one deletes an email, how is it managed in pst and ost files, i.e. is a part in the mid of a pst/ost file removed? If so, this would make the --append option not-applicable.
So, my question states, is it safe to use the --append option for backups of Outlook pst and ost files.
Related to this, Thunderbird saves emails to files (without extension) and besides them it writes some msf files which are much smaller than the former ones. Is it safe to use --append rsync option to backup the large files without extension (which contain actual emails)?