If you can receive E-Mail but can't send, it usually means that your internet provider is blocking you from using anyone's outgoing mail servers but their own. Many major ISPs are blocking outgoing emails. The reason they do this is to prevent you from using their internet connection for spamming purposes. You will still be able to send emails, but only by using your ISP'S outgoing mail server. Every ISP's outgoing mail address is different, but for the most part they will be mail.yourispname.com For example, if you had hosting with AOL.com, their outgoing mail server would look like this: mail.aol.com If mail.yourisp.com does not work and you are still unable to send emails, you will need to call your internet service provider to find out what their outgoing mail server is. Another reason could be that you have not checked the "server requires authentication" in your E-Mail clients option. In order to send E-Mails you need to have this ticked.
If you can send but can't receive E-Mail, you need to double check your password and your mail settings. It could also be that your domain is over quota, or your e-mail address itself is over quota. You will need to increase the amount of disk space to receive mail again.