If the SME is setup correctly, you would get an error like this if the Zimbra server thought that the email should be delivered to the SME server.
The log files on the SME or Zimbra server will indicate why the email is bouncing. Here's a wiki page describing how to dig info from the SME server's email-related logfiles:
http://wiki.contribs.org/Mail_log_file_analysisTwo possibilities occur to me:
- The SME is configured to use a DNS name or public IP address for the 'Zimbra' server that really resolves to the WAN IP on the SME -- so the SME just delivers to itself over and over (the SME server logs will show only local IPs when trying to relay an email to the Zimbra server).
-The Zimbra server thinks that email for your domain should be relayed instead of accepted -- that is, your domain name is not configured as a local domain on the Zimbra server (the zimbra logfiles should indicate that the email was accepted and relayed).