The issue was actually that we support many different protocols (not just mail) and some combinations of clients/protocols have had issues in the past (it might have been some FTP clients I think, but can't remember right now.)
Anyway, this restriction no longer applies as we now require server-generated app passwords for 3rd party apps: https://www.fastmail.com/help/clients/apppassword.html. So feel free to use as many spaces as you like in your password!
The issue was actually that we support many different protocols (not just mail) and some combinations of clients/protocols have had issues in the past (it might have been some FTP clients I think, but can't remember right now.)
Anyway, this restriction no longer applies as we now require server-generated app passwords for 3rd party apps: https://www.fastmail.com/help/clients/apppassword.html. So feel free to use as many spaces as you like in your password!