Not Allowed in header From

I have tried every configuration I can think of to make Contact 7 Form and Postman SMTP (v1.7) send email and ALWAYS get the same error: “Your IP: ::1 : Your domain {redacted}.com is not allowed in header From”

I have searched for hours trying to find this error and a fix for it.

The site in question has a third party email ‘processor’ and they tell me the email is not making their system (so is not being generated by the server) the hosting provider is telling me they aren’t to blame because the email isn’t processed by the server. ANYONE have any ideas how to make this work?? PS I have several custom coded mail forms that work perfectly on the site so it HAS to be the plugins