MAPI communication with Exchange 2010
If you've got an application that sends messages via your Exchange 2010 server, using SMTP, you might've noticed that things have slowed down a bit. The reason for this is because the Exchange 2010 receive connectors have a "MaxAcknowledgementDelay" setting, that will inform you if the delivery is successful, within a certain time frame. If the timelimit is hit, it then acks the submission.
To disable this you can set your receive connector not to use this feature:
Set-ReceiveConnector "Connector Name" -MaxAcknowledgementDelay 0
Further details are available on technet.