If you can receive mail but aren't able to send this usually means one of two things.....
Your internet provider is blocking you from using anyone's outgoing mail servers but their own or you need to change the SMTP outgoing mail port to 26 in email client software (Outlook, Eudora etc...).
Many major ISP's 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 usually it is mail.yourispname.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.
You can configure email software to set sender email something like email@example.com and use smtp account provided by your ISP to send email.