Restrict external IP when to domain is not local

[Exocomp]

How can I restrict by the source IP when the "to" email domain is not local server.

Example 1:
[*]zimbra server domain -> mydomain.com
[*]Someone from IP 111.111.111.111 want to send an email to user@mydomain.com

The above case should succeed.

Example 2:
[*]zimbra server domain -> mydomain.com
[*]Someone from IP 111.111.111.111 want to send an email to user@gmail.com

The above case should be matched against a white list of IP's that can send to domains other than "mydomain.com".

How can I setup something similar to the above?

[DualBoot]

You can add an IP into zimbraMtaMyNetworks parameter .
I think it is dangerous as you can make a mistake and run your MTA as an open relay.
SASL auth should always be used when sending from external IP.