Got an email from @Fastmail regarding the issue (https://
"Well, your message to support was not delivered to humans, because you first need to 'complete the support system' (wtf?), and to do that you need to log in, which you can't do because your registration wasn't completed, so basically screw you, you're not welcome in our 'customers first' universe!"
Well, I suppose it's a good thing my registration failed. Wouldn't want to trust this company with my email anyway...
Also on: @nekr0z