When setting up the Mail Server, you can use a user's long name, and are not limited to the eight-character short name.

When setting up the Mail Server, you can use a user's long name, and are not limited to the eight-character short name.
A user's long name is limited to 63 characters.

If you plan to use the long name you should only use alphanumeric characters, underscores, and dashes, which are 'safe' characters. You should avoid spaces, punctuation marks, and other 'special' characters.

Technically, the email RFCs permit spaces and many other 'special' characters, but typical conventions avoid them, so you should avoid them for best compatibility with other email clients and servers.