i can send mail from the server (using mail), and i can receive mail in my user mailbox on the server
but i can't send email using a client like thunderbird (it won't connect to the smtp server)
No offense, but Dovecot is not an MTA, it is a POP/IMAP server. It doesn't send email, so I'm not sure why you are posting this part of your problem to the Dovecot list - if your MTA is postfix, then you need to go there for problems with it...
and i can't download email from pop3 using thunderbird it always says authentication failed when i try to login
using squirrelmail, it always says username and password incorrect
but i know for sure the username and password are correct
dovecot conf file is as folows
-------imap_listen = [::] <------- -------pop3_listen = [::] <-------
-------imaps_listen = [::] <------- -------pop3s_listen = [::] <-------
http://wiki.dovecot.org/UpgradingDovecot
Scroll down to the part about setting up ports for listening.
Charles