"Disallowed email provider"

This is the only feedback when I try to signup with my own domain to mastodon.online

Can I get a list of accepted domains?

This is an annoyance because it messes up my system of keeping everything neat. I guess I’ll have to go through the longwinded effort of signing up a new gmail just to get access.

And I’m very surprised to not see this mentioned here on the forum as a sticky.

Each instance owner sets their own policies, so you’re best bet is to contact the admin of mastodon.online. A sticky about how instances are independently run and have their own policies could be useful.

As @weex said, it’s a per-instance thing. Also, I am pretty sure mastodon.online does not have an explicit list of accepted domains, but rather a list of banned domains.

Therefore, your email domain is likely blocked by mastodon.online. This can happen if your domain has explicitly been blocked, or you’re sharing an email provider with other blocked email domains.