If you are talking about partner mail. You have to dial into the mail system. Enter system admin: Type 99#, then 9(hidden),then password (default 1234). Eneter 4 for mailboxes, then ext. to be reset followed bt the #, then 3 to reinitial the password. This will reset it to 1234.
If partner mail password is not 1234, you will need to go through back door.