[claros]

Glen,
Stats works as you described...
But if you did a clean zimbra install (with no manual change on postifix config files) you are not an open relay.
Postifix itself default to relay localhost only unless you change your config...

But if a user of yours is sending messages through your zimbra smtp as an autenticated zimbra user with another sending address this is what you got in the report of most active sender.

This does not justify the recipient stats, so probably you have opened too much your network without authentication.
Check your logs under /var/log/zimbra and /opt/zimbra/log. If they are too big, you can check your logger database (where your stats came from....)

PS:
Sorry, i did not realize your "fetchmail setup"... it may be you are importing someone-else emails, check logs

Bye
Claudio