If you have a domain, you can have firstname.lastname@example.org as your email address. No problem there. You can set up as many of these as you want.
Behind the address, however, the mail must be routed to a specific mailbox. A mailbox is an endpoint for mail delivery within some local domain. If you want your mail to stay at JTAN, you will need a local JTAN mailbox. The name, or ID, of this mailbox does not need to match any part of the addresses that route to it.
Don't confuse mailboxes with addresses. There's no reason the two need to have matching names any more than your house number, street, or town need to match your given name.
All your mailbox ID's must start with your main username, followed by a dash, and some other word. You are only allowed one mailbox that has no dash, and that one must be exactly your man username. Keep in mind that all mailboxes on our side must live in the same local jtan.com namespace. Everybody wants a mailbox named "info", but really what they really need is an ADDRESS "info@theirdomain". The box can be anything.
In summary, you can have anything@yourdomain, but in the jtan.com domain, you can only have mailboxes email@example.com and firstname.lastname@example.org where user is your assigned JTAN username.
Have a look at the "Manage: Addresses" link in the upper right of your members page. The correspondence between your mailboxes and their addresses is shown pretty clearly there. This is also shown in "Manage: Mailboxes" from the opposite perspective.
Using a mailbox suffix (user-foo) that matches the user portion of the address (foo@domain) is a good convention.
Some folks would prefer a convention like user-1, user-2, user-3, and so on. In fact, it wasn't until recently that we even allowed mailbox ID names with the username prefix. Prior to May 2003, mailboxes were always serialized names like "pop1324". If you have boxes with the old style name, they still work, but new boxes must be created with the new name system.