By default, your domain will accept email only to the same address as your tentacle.net username. For example, if you have registered example.com, and your normal email address is joeuser@tentacle.net, then you can only receive mail addressed to joeuser@example.com. This will end up in your joeuser@tentacle.net mailbox just as if it had been sent there directly.

You can add just about any address you like (though certain addresses are overridden, so you can't use postmaster or abuse, for example) as an alias, which will behave the same as above. If you want to change this mapping, you can edit the file in /etc/mail/exim/virtual with the same name as your domain.

In addition to having aliases just to your mailbox, you can set up any number of aliases pointing elsewhere. Using example.com again, you might see something like this:

joeuser: joeuser
janeuser: janeuser
admin: joeuser
albert: albert@somewhere.com
mildred: spam@anotherplace.com
friends: joeuser, janeuser, test@domain.com

Note that leaving off the domain on the portion to the right of the colon denotes a local, tentacle.net address.