Stalw.art allowed me to create an email with spaces. Thunderbird also allowed me to send to the address, even though it has a warning symbol and says "[...] is not a valid email address".
And to my surprise it was also delivered sucessfully.
Edit: I also wanted to see if Lemmy allowed spaced emails. It does not :(
Haven't heard about the "PolyForm Strict License" before. It looks to be a young project, yet feature rich.