This error message usually means that the connections to the recipient mail server have exceeded the limit. This high traffic to mail servers can sometimes be a temporary problem due to a spike in the connections.
This condition can happen if your mail server is setup to only allow one connection to it per IP and you have Campaign Enterprise sending several campaigns at a time. To solve this, in the Mail Server Management area, edit the mail server, go to options, and set the concurrent sending to "1 Thread". In this way Campaign Enterprise will queue up the campaigns and only send one at a time.