This can be very handy when trying to troubleshoot.
Firstly you need to have the Telnet Client installed on your server.
Cant remember if this is under Roles or features but it is there.(Win 2008 R2)
After you added this open you command prompt, probably best to do it in admin mode.
Then you type the following:
Telnet 10.10.10.10 25 (So basically that is pointing to the ip on port 25. Change accordingly.)
Mail From: firstname.lastname@example.org
RCPT TO: email@example.com
To start with the body of your mail type
If you want a Subject:
Subject:-*Type Subject* Press Enter Twice
*Now type your Email*
Once your email body is done type a "." in it's own line
Now you should get a message saying that your mail was send...
If there was issues you should have received a error by now. Hopefully that error is useful.
Update: Much easyer to do this via PowerShell These days...: