Test email 'Not Valid'
If I change site2 to send the test to my personal email instead of work then it arrives.
Obviously my work email is valid, it receives email every day and site1 sends a test just fine. Neither site domain is the destination (work) email domain.
All the sites are .co.uk but company domain is .com... that's what happens when you do stuff at the end of the day with your brain frazzled!
Unsurprisingly it works fine with the CORRECT domain !!