[exim] Trying to add alias users to the same accounts

Hi,
Can I add an alias user to the user database. My account on my server
is robwilson, but I want it to be rob.wilson. I imagine it's not as
simple as just changing my username in the userdb file as that is
nothing to do with the user ID in the actual operating system.

Please also be aware I am something of a noob with E-Mail servers. How
do I make it as secure as possible? I know that making it totally
secure would mean closing the port son my firewall but that defeats the
object somewhat.

Any useful word gratefully recieved.

Regards,

Rob.