Hi marcus_a,
Thank you for your post, and I regret the delay in my response.
Generally, a rejected diagnostic code indicates that the email has been rejected at the recipient's end. After checking, I can confirm that the email is not blacklisted on our end. I recommend trying to send an invoice to yourself. This will allow you to verify the email address from which the email is being sent. Once you have this information, you can provide it to the recipient. This will enable them to whitelist it against any anti-spam systems they have in place, as well as add it to their email address book.
By having the email whitelisted and included in their address book, it should help to prevent the email from being incorrectly marked as spam.
I hope this helps! Please feel free to reach out if you need further assistance.
Cheers,
Princess