[bad_user]

Without a good importing tool (warning: the GSuite importing tools are shit), I recommend "imapsync":

https://github.com/imapsync/imapsync

After cloning the repository and installing the listed dependencies (some Perl packages available in your Brew / Ubuntu repository already), you can do:

    imapsync \
        --host1 imap.old.com \
        --user1 "[e-mail]" \
        --password1 xxxxxxxxxxxxx \
        --ssl1 \
        --host2 imap.new.com \
        --user2 "[e-mail]" \
        --password2 yyyyyyyyyyyyy \
        --ssl2 \
        --errorsmax 1000000 \
        --syncinternaldates \
        --useheader 'Message-Id' \
        --noreleasecheck \
        --noexpunge \
        --automap 

Works great for importing to and from Gmail, but you might want to add a folder rewrite for the Sent folder (with --f1f2).

But with FastMail, for those interested, the out of the box importing tools in their web interface work great, so no need for any command line tools.

[teh_klev]

I've used imapsync in the past to migrate thousands of mail accounts from and old decrepit IMAP environment (with some weird edge cases) to a shiny new IMAP based mail platform.

It's a hugely flexible and versatile tool and it's quite zippy as well, can highly recommend.