I get the following error only when replying to received emails and only from one domain, mail.com. If I create an email and send directly to the mail recipient every thing works correctly.
Delivery has failed to these recipients or distribution lists:
Microsoft Exchange has been trying to deliver this message without success and has stopped trying. Please try sending this message again, or provide the following diagnostic text to your system administrator.
The following organization rejected your message: mail.com.
Diagnostic information for administrators:
Generating server: cdptpa-imta16.mail.rr.com
mail.com #<mail.com #4.4.7 smtp; 250 2.1.5 <firstname.lastname@example.org>... Recipient ok> #SMTP#
The generating server is a smart host that we forward all mail through because our ISP can't seem to properly create a PTR record for us.
Sounds like it is on their or the ISP side and not mine.