Greetings One & All

We're trying to troubleshoot an issue with auto-forwarding (hidden from user) apparently not being delivered.

Forward target is on another mail server managed by others. Mail sent direct is successfully delivered.

Target is one of two (hidden) forwarding addresses for this user.

Q: Where/how can I find a log of the forwarding activity? So far I've tried ...

# tail -f /opt/zimbra/log/mailbox.log
and ...
# grep [some-string] /opt/zimbra/log/*

Thanks for reading.