LPT: If a site doesn't accept alias sites like Simplelogin, create the account with a disposable email site then change it to your alias
I just did this on a website that said my Simplelogin alias isn't allowed for signup, but changed it successfully after the fact from a disposable email.
I do the same but it doesn't always work.
I forget which site it was but there was one I ran into that checked the dns records.
It blocked any domain that points to simple login servers.