The exchange server is bouncing back emails send with the error message organization error: A communication failure occurred during the delivery of this message. Microsoft Exchange will not try to redeliver this message for you. Please try resending this message later, or provide the following diagnostic text to your system administrator. The following organization rejected your message: VA3EHSMHS004.bigfish.com. I restarted the server , but still no luck

Try the forums under the Exchange category on technet. This forum is for help in writing power-aware software. The following is signature, not part of post Please mark the post answered your question as the answer, and mark other helpful posts as helpful, so they will appear differently to other users who are visiting your thread for the same problem. Visual C++ MVP

Hi, If it returns back to 5.7.1, mail sent the company is blocking your mail.

Fatih is 100% correct - are you sending from a test system, or from a corporate system? If the former, many corporations (and many Internet operations, such as Craig's List) will block email they believe are sent from IP address ranges that are ISP DHCP ranges. If the latter, it is possible that your corporate SMTP system has been block listed by the target system. You will need to work with the owners of that system to determine why your system is block listed.