I use gmail as a mail dump:

• ‘current’ and important mail is kept on my fastmail.fm account for access from everywhere
• ‘old’ mail (>~12 months) from Sent and Trash are semi-regularly transferred to gmail where they can be searched etc whilst freeing up space on both my local machine and fastmail.fm

This means I have to transfer quite large volumes of mail every few months, importantly, keeping all tags, header info etc intact.

For this I use larch and run the following code either locally (with a good internet connection) or on a remote server (usually having fast connection).

This is specific for fastmail, so just change the server information over and the password. Yes, this probably isn’t very secure what with the passwords sitting in a shell script. YMMV.