[Dovecot] duplicate all emails for 'backup' after user deletes them?

I have a user who occasionally reads, then deletes emails using his phone, on his and others email accounts, causing emails deleted on mail server, then he says 'oops, can you recover emails I've deleted?'

what's the best way to duplicate all emails to duplicate accounts ? backup folders ? so, next time he deletes emails they can be 'recovered' on the server ?

I was thinking along the lines of duplicating ALL emails...?

any tips and pointers appreciated

-- Voytek