When performing test on a Microsoft Exchange server, some tests will return a transaction time warning. For the most part this is not an issue and nothing needs to be done. This delay is intentional on the part of Microsoft Exchange to help prevent spam. The warnings are typically given when it takes more than 1 second for the SMTP connector to respond to the request. This process of delaying multiple mail requests is called tarpitting.
If this warning bothers you, there is a change that you can make that will fix the Transaction time warnings for Exchange 2007 and 2010.