Getting “recipient-domain-mismatch” Errors – Email Send Rejected

All my DNS info seems to be good – passes Mandrill tests. The plugin seems to be configured properly. API key is fresh and working. But I get “recipient-domain-mismatch” errors on every send attempt and I really can’t tell from the logs (using Query Monitor) what the problem is. I’ve tried sending to mac.com and gmail.com addresses so far, with the same results.

Further, these errors are using the API, and my host does not allow Port 587, even as a fallback, and that can’t be changed in Mandrill (that I know of), so I’m forced to use the API. Oddly, when I send mail via my Contact 7 form, the mail send works (maybe it’s using PHP mail because it goes out with a different sender address)! But any mail using the plugin test forms and notifications on Woo orders will all fail.

Any ideas or something specific to look for in the Query Monitor to figure our why this isn’t working?