Create nicely rendered PDFs of all your email for archival purposes.
Your email is valuable. The format that it is stored in with Apple Mail is neither future-proof nor accessible. Use Email Archiver to create an archive of all your emails, or just a few folders.
Creates a hierarchy of PDFs, one for each email, using the subject line as the filename, with all email headers included. Attachments are in a folder alongside the email.
If you make a slip while setting up IMAP or POP a lot of email can be erased. This helps with that problem.
Also, if you want to store all your emails on a file server somewhere, be it S3, FTP or SMB, this will help.