> I use their lowest paid plan, $5/year, so I've set up Thunderbird to download messages immediately, so as not to bust my storage limit... I have my own domain, and I just point my domain to fastmail now instead of gmail.

The plan descriptions on the fastmail.fm web site make it sound like you have to sign up for their $40/year "Enhanced" plan before you can use your own domain. It's still a good deal, obviously, but not as good as $5/year.