503 you are already authenticated
I have noticed a problem with the combination of POP-before-SMTP and SMTP-AUTH being enabled at the same time when using OS X's Mail application. This may also affect other clients but does not appear to affect Outlook.
Using tcpdump I can see that when configured for SMTP-AUTH, OS X Mail will always try and authenticate. Other clients such as Outlook only authenticate if the server requests it.
This conflicts with POP-before-SMTP because if OS X Mail tries to authenticate again via SMTP-auth when it is already authed via POP-before-SMTP, Qmail will give a '503 you are already authenticated' error and terminate the SMTP connection.
Has anyone else experienced this with OS X Mail or any other client? I suspect the only workarounds may be to disable POP-before-SMTP auth or to recompile qmail-smtpd to not drop the connection on duplicate authentication.