Not exactly on the Open Source side, but the tool is very open source. It pulls all the emails as .eml files
I had managed to break things royally with my hosted domain and spent much of the weekend dragging IMAP folders around till I discovered this little gem.
Links of Note:
The other fun tool of interest is: https://forwardemail.net/#/ This lets you have your own domain and use a regular gmail to read receive them. Ie. email@example.com and firstname.lastname@example.org all goes to email@example.com.
If you’re hosting your own email, then none of this is really relevant, but thought I’d share since this saved me so much time this weekend.