Can you see what the error message is when you try to send the email?
The image you posted previously suggested that you are trying to send an email using the 'mail.isp.com' which, if you are trying to send it via the gmail account, is obviously the wrong one.
It is odd that you are able to receive the emails but not send them, and I would imagine that it has something to do with the settings that you are using.
could you tell us what the outbound settings are?
-Scott-