auth between postfix and dovecot?

[alice]

hello experts,

I have installed postfix and dovecot in the same machine.
Their configure looks as:

service lmtp {
   unix_listener /var/spool/postfix/private/dovecot-lmtp {
     mode = 0600
     user = postfix
     group = postfix
   }

unix_listener /var/spool/postfix/private/auth {
   mode = 0666
   user = postfix
   group = postfix
 }

 auth_mechanisms = plain login

 !include auth-passwdfile.conf.ext



my question is:

when postfix talks to dovecot, does it require user's username/password 
for authentication? or this communication just goes without authentication?

I asked this, b/c my webmail send mail from localhost has been going 
without authentication to postifx. so i am not sure if postfix talks to 
dovecot without requiring auth too.


Thanks
alice