If you can receive email, but you cannot send email it is possible this issue lies with your ISP. Many broadband ISPs block the SMTP port 25 to reduce or eliminate spam being sent through their network.

First, you should try changing the SMTP port in your mail client to 26. We have enabled this on some servers for this very reason.

If that does not work, please try changing your SMTP server to match your ISP's outgoing mail server. Even though your email address is not through them, you can send email through their outgoing mail server. This is the best way to do it, as your ISP's SMTP server should never fail.