Currently Being ModeratedJun 12, 2013 12:33 PM (in response to paulhutch)
There's a whole lot of details and info missing here and honestly you'll probably have better luck asking in the Mail forum.
Under Mail > Window > Activity and see if any error messages pop up in the window while sending. You will have to turn your SMTP back on again to test.
Mail > Window > Connection Doctor is another tool you can try that should show exactly where the process is failing.