First check to see if the bounced message contains an error message like:
Undeliverable mail. Mail relay not allowed by email@example.com to firstname.lastname@example.org. PLEASE NOTE: You may verify your email address in order to send mail to email@example.com by clicking on the following link or pasting it into your web browser: http://www.ministerschedulerpro.com/netservices4/verifyEmailSender.php?fwd=3275&email=joe%40hotmail.com
If the bounced message contains such an error message, then the error is being generated by our spam-protection system. These errors occur because the ministers are replying from a different address than the one at which they were originally sent the email. They can verify their address and then send you mail by clicking the link that is provided in the bounced message or pasting it into their web browser.
If there is no such error message in the bounced message, then there could be numerous other issues at play. We need to see the bounced message to determine the exact cause of the problem. Forward us a copy of the bounced message at firstname.lastname@example.org, and we will take a look at it and get back to you with an explanation.