Backups are like snapshots; while you can individually walk through every second of 'restoreToTime' you may still miss content and especially sent mail if they don't save to the sent folder.
Just so you're aware, the NE version has an excellent add-on that does envelope forking called
Archiving & Discovery w/ cross mailbox search
often referred to as
ZAD for short - more on it:
/pdf/Zimbra Archiving and Discovery Whitepaper.pdf /pdf/Zimbra Archiving and Discovery Release Notes.pdf
For FOSS, many use the
always_bcc method of archiving.