If you try to sent an e-mail using webmail and you get the error...

Requested action not taken: mailbox name not allowed
Server replied: 553 5.3.0 ... No such user here

Go to 'Options', 'Personal Information' in webmail and complete your 'Full Name' and 'Email Address' (or at least the name and e-mail you would like shown on your sent e-mails).