Thanks for the vote of confidence! If you have a custom domain at fastmail you can avoid any lock-in by using it for your masked addresses. Settings -> Domains -> Team Settings -> Masked email domain.

With that it's entirely portable. You can point your mx records at any other provider.

Disclosure - I work at 1Password, though I had only tangential involvement in this effort.

I second those thanks. We very much appreciate your confidence in us!

Disclosure - I run Fastmail, though also only had tangential involvement in this effort.

Can I generate masked emails without fast mail? I already have a domain setup

This feature is an integration with Fastmail's masked addresses. You don't need to use their domain, but you do need to use them as your email provider.

Disclosure - I work at 1Password, though I had only tangential involvement in this effort

As mentioned above, but which I think is a point worth being made several times, because I really think they've done a lot correctly here:

You can use your own domain.

Nah, you can set up a catchall on almost any provider and generate your own random usernames.

Fastmail doesn't (and, due to their model, can't) encrypt your email at rest in a way they can't read it.

I can't recommend using an email system like this to anyone who cares about security or privacy.

Fastmail admins can themselves read your entire email history, as well as any law enforcement fishing expedition in US or AUS.

That's true, but hardly unique to them; proton is the only email provider I know of that even tries to cover that case.

Hushmail did it years ago, until they were eventually forced to backdoor their client to steal a surveillance target's pass phrase.

Law enforcement or fastmail admins reading my personal email isn’t something I want, but isn’t so distasteful as for it to really be in my threat model such that I’m willing to go the lengths required to get an email provider that doesn’t have this ability, such as proton mail. I care about security and privacy and recommend fastmail, but I could see someone whose end all be all criterion of interest being privacy not wanting to use fastmail. But, this isn’t 99.999% of the population.