[jrimbault]

Just this afternoon I was with my father looking for a way to backup all of his emails on his yahoo account.

I was looking "naïvely" for the button to request all of his personal data. I didn't find one and there's probably one somewhere I'm guessing.

I resigned myself to set up Outlook on his computer and make a manual backup.

[idorosen]

I use getmail (similar to fetchmail) to routinely archive (i.e., sync without deleting + reindex) all of my emails from various free accounts, just in case. It can save to mbox, Maildir, mh, and other formats that are easy to import to any MUA/LDA. This is worth doing for all e-mail, and I have a patch to make it support OAuth. I don’t think it supports JMAP, but it’s great for IMAP, Gmail, and Yahoo mail and deduplicates messages by ID and content, etc. while preserving tags/mailboxes (if saved as Maildir). I highly recommend running something like that in a cron job somewhere once a week to sync locally with some sanity checks (e.g., did it save any new messages? did the folder grow? Etc.)

I also use it to save Spam/Junk folders, which then comes in very handy to train my local spam classifier for my self-hosted mail servers with lots of data. (Over 3TB of spam saved so far and about 20GB of ham.)

Gmail’s spam filter has had a higher false positive rate than usual for me lately, so I have a little report emailed to me once a week of likely ham in my gmail spam box, which has found at least 3 messages per week that I missed.

[davchana]

I use a mix of google email labels, apps script, spreadsheet & drive folder to download every email (older than 15 days, so that I have enough time to delete it) as .eml files in Google drive folder, which by turn downloads it to my local disk.

The spreadsheet keeps log of each msg in a thread.

Labels marks the downloaded emails.

Apps script run on a trigger & does the heavy lifting of actually downloading the .eml.

[reid]

Yes, I believe a local IMAP backup is the way to go.