Answer
Yes.
As per RFC 2822 we do support "!" and "#" in usernames.
Please note: The character "!" is an old fashioned domain separator before widespread use of the "@" symbolwas adopted. If a username is created using the ! character, this user will have to log on to GMS Webmail using user!name@domain.dom This is because user!name is interpreted by GMS Webmail as name@user.
See Also:
Keywords:usernames support !