[precurse]

Google Takeout works really well, but you can also connect using IMAP or POP3 to download all your emails with an email client like Thunderbird.

Then, if you've setup a new email account with another provider, you can connect using IMAP and copy your emails that way.

https://support.mozilla.org/en-US/kb/thunderbird-and-gmail

[sufehmi]

POP3 always fail when you have so much emails: https://news.ycombinator.com/item?id=30485039

IMAP only download the index - everytime you click on an email, it'll then download it. When you have 150K emails, then you need to click on all that emails 150K times.