help to setup zimbrabehind an external gateway

[hcmnttan]

Hi there,

I am setup a zimbra open source server behind an external gateway. And this is how I setup:
- on zimbra : config MTA for external delivery --> point to gateway IP
- on gateway: forward all emails that send to my domain to zimbra IP
- config mx record point to gateway ip
- on zimbra: setup an internal dns to that it will resolve domain mx record to it's local IP

After this setup I could test sending email in/out OK.
But the 2 problems occur is that:
- I found emails forwarded from gateway to zimbra server sometime detected as spam.
- When I try to telnet smtp to zimbra to send an email from someaccount@mydomain.com to someaccount@mydomain.com without any authentication. It sent successfully.

So the question is:
- can I config zimbra spam service to trust all emails that coming from my gateway IP ?
- can I enforce all smtp connections to zimbra must be authenticated ( accept my gateway IP) ?
- does zimbra has a best practise for this case ?

Thanks for any help.
Tan