[Dovecot] login with alias

[Gasha]

Hi all,

i'm setting up mail server and here is the question:

if, for example "firstname.lastname at domain.com" is e-mail address, then

how can i setup login as "firstname.lastname" and "password" not only 
"firstname".

Of course, users want all things simple. But i don't want to use login 
names longer than 8 chars.

Any ideas? Or should i try to create users as "firstname.lastname" in 
/etc/passwd file ??

Gasha