SMTP] Cannot send message (Sending message “e-mail title” failed with error: Email body not accepted due to the following reason: "5.7.1 Reject for policy reason
That error message “body not accepted policy rejected sending error” sounds like there is something in in the email that is deemed as possibly spam or violates the server sending policy.
So you will need to contact your SMTP mail server technical support and tell them you are getting
5.7.1 Reject for policy reason when sending those specific emails. They will then advise on that.