The current implementation (1.4.6 and I think all version before) has a bug in the way the Base64 encoded username/password combination is generated. There needs to a null character before the username as well, and not just as a separator between the username and password.

http://qmail.jms1.net/test-auth.shtml

The fix is to modify line 239 from

credentialsEncoder << username << '\0' << password;

to

credentialsEncoder << '\0' << username << '\0' << password;

Tested with my postfix server using AUTH PLAIN and the above fix.