/usr/local/psa/var/log/maillog
/var/log/maillog (may not be present)
/var/log/messages (just in case)
Let me see if I understood correctly: email account was setup as mailbox and with forwarded email address? Test email reached the mailbox, and you were able to see it in webmail? It just never got forwarded to either the local other email, or external other email?