I know you asked for a simple solution NOT requiring a full-blown IMAP server, so I'm ready to get plenty of negative votes for my answer. :-)
Cyrus IMAPd is one of the most feature-complete open source IMAP servers around. With its ACL features you could first create a normal account for the duration of the backup, and when ready, just drop the write/delete access from your user account, so the mailbox will effectively be an archive folder with no way to accidentally delete your archived messages - at least not via IMAP.
Old and not so good, but a simple POP/IMAP server, uw-imapd can be more like install-and-forget solution, too. Just transfer your mail over IMAP to it and then your mail is accessible via IMAP or just by browsing /var/spool/mail/youraccount file. By making the file read-only with
chmod 400 /var/spool/mail/youraccount the mailbox would effectively be a read-only mailbox.
Dovecot is also a quite simple to setup and more secure & feature-complete than the uw-imapd I actually hate.
Anyway, I would install some IMAP server even if the initial setup can be more tricky. With your own IMAP server it's simple to add new accounts and archive more mailboxes, and you can reach the mail via several different methods; the mail client of your choice, via webmail if you install something like Horde, SquirrelMail or Roundcube, or even via the raw mail files.