If you are unable to send emails first double check all of your email settings here
Other common issues are:
503 Valid RCPT command must precede DATA
Make sure you have authentication turned on for sending emails too. This is usually under the More Settings or Advanced Settings of your email program. The setting will be worded simular to ''My outgoing server (SMTP) requires authentication'. Make sure this is enabled and you either select 'use incoming mail server settings' or you type in your email address and password
Email Server is not responding or cannot be contacted
This can happen if you are using the wrong port numbers. Make sure you have used the correct server name and the correct port number for both sending and receiving.