Mail Server 421 Errors - Too many concurrent connections

Too many concurrent SMTP connections errorMany mail servers are not setup for mass emailing activities The error message in an SMTP conversation of ‘421 Too many concurrent SMTP connections; please try again later.’ can mean a lot of things.
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.