As a consultant I often test SMTP connectors on Exchange.
There are several ways of doing this. There is the good old way of using telnet, ExchangePro has a great article describing the Telnet way of testing you SMTP connection:
The other way of testing if a SMTP connector works on Exchange (2007/2010), could be using PowerShell. In PowerShell 2.0 there is a builtin cmdlet cmdlet: Send-MailMessage
You can run the following command in PowerShell to do some mail testing:
This only works for PowerShell V2, since the command is now builtin.