Error: 400 Remote server unavailable at this time

I am using the SMTP scanner and when a sender attempts to send us mail, they receive this error. What does it mean?

This error occurs when the SMTP scanner cannot connect to your internal mail server. This could be because the internal mail server is offline, the mail application is experiencing issues or the traffic is being blocked. The SMTP scanner, by default, acts as a proxy between the internal mail server and the outside world. When the internal mail server is unavailable, the SMTP scanner will reject incoming mail for that domain and immediately inform the sending server of the issue with the error: 400 Remote server unavailable at this time. To correct this issue, mail flow must be restored. Investigate why the internal mail server is not accepting mail. As stated above, this could be due to a disruption of service on that internal mail server or a disruption in communication between the server running the SMTP scanner and the internal mail server.

This article was originally published in the GWAVA knowledgebase as article ID 1233.