Default Receive Connector Timeout is 10min. This is enough in most of the situations however in case of slow / unreliable connection this could cause delivery problems, especially for large emails.

You may get 421 4.4.1 Connection timed out errors in SMTP Receive Connector Protocol logs.

You can increase the timeout using Exchange Management Shell

To check timeout for all SMTP receive connectors:
Get-ReceiveConnector | fl Id*,*connectiontime*

To check timeout for a specific SMTP receive connector:
Get-ReceiveConnector “*ConnectorName*” | fl Id*,*connectiontime*

To set timeout for a specific SMTP receive connector to 30min:
Set-ReceiveConnector “*ConnectorName*” -ConnectionTimeout 00:30:00

Exchange Server 2007
Windows Small Business Server (SBS) 2008