I am very new in server management thing. I want to make email accounts like firstname.lastname@example.org. I know something to do with Sendmail thing , but dunno what ?
I am on Ubuntu 10 server.
There are some good walkthroughs on setting this kind of thing up at How To Forge. I wouldn't follow any of them exactly since they won't perfectly match your requirements, but this one seems pretty much what you are looking for.
Personally I'd go for Dovecot over Courier, though only because it's what I'm used to. So maybe this one, though that's using an LDAP backend and is for 9.10.
Anyway, those two should at least give you an idea of the steps involved - even if they're not exactly what you are wanting to do.