Zimbra - Add Trusted Relay to SpamAssassin

My domain name registrar offers email aliases and forwarding; however, this feature causes my mail server's SPF checks to fail because the registrar's mail relay sends on behalf of the originating server.  I fixed it by white-listing the registrar's mail relay IP block in SpamAssassin.

1. Add a "trusted_networks" line to the SpamAssassin config.  As of Zimbra 8.5 and later, the reported location to customize SpamAssassin is here:
/opt/zimbra/data/spamassassin/localrules

I refer to this article for the latest location for customizing SpamAssassin: https://wiki.zimbra.com/wiki/Anti-spam_Strategies#Customizing_SpamAssassin

Edit the file in here called local.cf and add your "trusted_networks" (example below).  You'll notice this file contains a comment at that top that says something like, "This is the place to customize SpamAssassin..."  I've also seen a file called user_prefs.cf being used instead.  To be honest I don't think the file name matters much because I believe SpamAssassin reads all the cf files in this folder.

# TRUST REGISTRAR RELAY
trusted_networks 123.45.6/24

2. Restart Zimbra.
su - zimbra
zmcontrol restart

FURTHER READING
http://spamassassin.apache.org/full/3.4.x/doc/Mail_SpamAssassin_Conf.html