Well, a couple points: 1. It depends if you don't trust them in the long term or the short term. In the long-term, after you finish the migration, there's no n

Well, a couple points: 1. It depends if you don't trust them in the long term or the short term. In the long-term, after you finish the migration, there's no need for Fastmail to still have access to your account, and Google does let you revoke/cycle app passwords, so I would just do that and that would cut off Fastmail's access, even if they did retain your password (which, based on what I know about them, I doubt they'd do, but I don't know them personally). 2. For the actual transferring, if you don't want to use Fastmail's tool, I'm a big fan of imapsync, and my second best choice would be offlineimap. I like the former for my needs, where I do a lot of one-shot migrations/backups/restores, but the latter does have the benefit in your case that it can be run as a daemon, and then it'll slowly steadily upload your mail in the background over the course of however many days it'll take. 3. It should not take 7 days to move 15GB. I've done it in under 48 hours. Part of the bottleneck might be that your local machine/network/etc. I would try to fire up a VM with a cloud provider, preferably one of the fast-network variants, and run imapsync from there.