Thanks for the info. I looked at the site settings. domain3.com was a co-domain on another site. I had a www version setup (so www.domain3.com
was listed in local-host-names), but no non-www version (so domain3.com was not listed there). I went ahead and set up domain3.com (no www) as a co-domain and it's now showing up it the local-host-names file. I did a postfix reload and I'll keep an eye out for any more errors.
How should I set up co-domains if they aren't going to be used for email. My only options are local mail server or external mail server ... there is no "none" option.