I thought that I had this under control before. But for some reason during our last email marketing promo, I start receiving from our mass email client (built in house)..

The message could not be sent to the SMTP server. The transport error code is 0x800ccc67. The server repsonse was 421 4.3.2 The maximum number of concurrent connections has exceeded a limit, closing transmission channel

again. There's several places I've checked to make sure that wouldn't be an issue. First I checked that receive connector was set to receive an adequate number of connections on our relay connector (1000 connections). Then, I would later find out about Throttling Policies. I created one and set all the properties I knew to set in terms of the policy following properties to 1000; EWSMaxConcurrency, OWAMaxConcurrency, CPAMaxConcurrency, and CPAMaxConcurrency. Still, the email client starts receiving the error shortly after 100 has been sent and takes about 15-30 seconds. The process is then repeatable, but still the error gets received at the same spot everytime.

Is there a rate setting that I am missing? Was there a windows update that I missed looking at? Should the software have it's own throttling feature?