I'm going to bump this topic, since I'm having the same problem. I'm using CENTOs 5.2, and the installation went fine. I can send email unauthenticated through telnet, and it gets sent from the server and is received at the server in the proper maildir (only from telnet).
But I cannot authenticate remotely using thunderbird... I get these errors:
Nov 10 19:29:56 newinstance postfix/smtpd: sql_select option missing
Nov 10 19:29:56 newinstance postfix/smtpd: auxpropfunc error no mechanism available
Nov 10 19:33:58 newinstance saslauthd: do_auth : auth failure: [user=jon] [service=smtp] [realm=mydomain.com] [mech=pam] [reason=PAM auth error]
I updated my smtpd.conf file with the one posted here, but it did not help.