This field allows you to specify the name of the user's mail box. This is the actual name of the mail box on the Server and is used by e-mail software to connect to the mail box.
This is quite separate from the user's e-mail address. For example, the user John Smith might have the mailbox johns but the E-Mail Address John.Smith. The user can actually receive e-mail under either of these names, but it is generally considered good practice to keep them different so that a person outside the company can know a user's e-mail address without knowing their main box name. This helps keep the mail box more secure.
Allowable characters are limited to 0-9, a-z