I don't know anything about iRedMail, but my guess would be that it has configured Postfix to do content scanning during the incoming SMTP transaction. If so, Postfix can't close the connection early because the client will have no idea whether the message was successfully delivered.
For a standard Postfix install, I would look at the configuration and move any intensive content-scanning activities to occur after a message is accepted.
However, web control panels like to do things their own way, and tend not to behave well when manual configuration changes are made. I would suggest asking in the
iRedMail forums rather than here.