"Finding your mail files in Evolution is pretty straightforward
if you know where to look. The default path is:
In the "mail" folder you’ll find bog-standard .mbox files
which you can import into any other email app worth its salt.
Evolution also has a nifty backup and restore feature that will
compress your data (email + addresses, calendars, contacts, notes
& to-dos) into a handy (for Linux users) .tar.gzip file --and
restore a full data set from the same file."