I worked professionally on email clients for 4 years, and was a hobbyist for a while before that. I almost always use imapsync by Gilles Lamiral.

By default it will do a uni-directional transfer, but it accounts for duplicates and can be instructed to delete emails that don't exist on the originating side to have a more sync-like behavior. It also has a variety of filtering tools built in (ie. only transfer last week's emails, or only emails with a particular header).

If I sound like a fan-boy, it's because I am: I would not have been so comfortable in my work without having solid backs, it's saved me literally days of coding up specialized tools for various tasks and repairs, and saved me from dataloss more than once.

http://imapsync.lamiral.info/

My workflow was to run a VPS with Dovecot and pull my email from my various accounts on to that, and then put the Dovecot maildir under git, and then occasionally bundle it up into a 7z archive with max compression (nowadays, I might look into xz) to pull onto an external hard drive.

Fanboy is what I wanted in a way, someone who knows a system in and out, and like me has run mail systems in the past.

If you know this well, and it is working for you, and you used it to dev clients, then that says a lot, as if you are dev'ing a client, you need a defined corpus and easy way to roll in and out of certain things you have done.

I wish I had this when I was trying to make a webmail based IMAP reader in php.

I use offlineimap. It does incremental syncing, and bidirectional syncing. You can pull email from one server and then later upload it all to another if you switch.

offlineimap.org.

I use CloudPull, a Mac app, to backup all my Gmail emails on my local hard drive. It also backs up my Google Contacts, Google Calendar, and Google Drive. (My local hard drive is then backed up in two ways, one local, one in the cloud.)

I used to use Backupify, a cloud-based service to backup my Google Account, but CloudPull is less expensive, and more in my control.