right click your account in the folder pane and select settings. In the setting pane that opens check the field Email address: is complete, correct and does not have a blank space either before of after the text. Is the description of the SMTP server show at the bottom of that pane consistent with the emails address. for example your address might be billy@ comcast.net is the SMTP server with comcast? Click the edit button and check the user name. is it the same as the one used on the incoming account? (you will need to go to server settings on the incoming account to see the user name)