There's a limit of 100 domains, plus 1 for every user in the account. https://www.fastmail.help/hc/en-us/articles/1500000277382-Account-limits https://www.fast

There's a limit of 100 domains, plus 1 for every user in the account. https://www.fastmail.help/hc/en-us/articles/1500000277382-Account-limits https://www.fastmail.help/hc/en-us/articles/1500000277382-Ac... Probably mostly there to prevent rampant abuse from spammers, scammers, or people just doing really weird shit creating millions of $thing where everyone else has a few dozen or hundred.