I have a Small Business server setup with about 23 users. We are able to send email to anyone with the exception of any user at email@example.com. I have a Reverse DNS setup pointing to our server. I checked the blacklist and we are not blacklisted. The message that I get is similar to this
Could not deliver the message in the time limit specified. Please retry or contact your administrator.
Any suggestions on how to troubleshoot this issue is greatly appreciated. Also, our ISP is Bellsouth.net, I changed the configuration from DNS to smart host (mail.bellsouth.net) and then no email goes out at all. As soon as I put it back into DNS, all of the email goes out with the exception of pending email to Bellsouth.