When the Server queue contains several messages to be directed to the same domain, the SMTP module opens a connection to that domain mail server and sends messages one by one. If the established connection is slow and there is a large message in the Queue, other messages would wait too long before being delivered. You may want to allow the SMTP module to open additional connections to the same mail server and send other messages in parallel.
Channels/Host: Use this setting to limit the number of TCP connection the SMTP module is allowed to establish with one domain mail server.
Add Channel after: When the SMTP module cannot send a single message within this time period, and there are other messages in the same Queue, the module opens a new parallel connection.
when Queue Size is: If the SMTP module is sending a message, and there number of unsent messages in the same Queue is equal or larger than the value of this setting, the module opens a new parallel connection.