The Queue Manager service allows you to control the flow of messages through the mail queue. This service takes the place of SMTP32exe. by delivering messages to both local and remote destinations. Although the SMTP32.exe program still exists, it simply informs the Queue Manager when a message requires delivery.
Note: When sending mail, if a valid 1xx or 2xx response is not received when connecting, the Queue Manager will roll to the next MX record.
The mail queue is also known as the spool is a directory that stores mail messages that are waiting for delivery. Files in the queue include incoming messages, outgoing messages, attachments, and error messages.
The queue manager releases messages one at a time in the order that they were received.