I can successfully receive mail but I am not able to send it.
One possible reason may be SMTP Authentication. SMTP Authentication checks to see that you are authorized to send email before allowing you to relay messages through the server. This means you MUST configure your email client to authenticate with your username and password.
The reason we use this system is because if the server did not check before allowing you to send email, then anybody on the Internet could send email through the server, thus leaving it open to bulk mailers (Spammers).
NOTES:
1) Some ISP's force you to use their SMTP servers to send mail by blocking your ability to make connections to external SMTP servers. Please check with your provider to see if they employ "port 25 blocking".
2) If you are connect to a local net work with Firewall, kindly check with the Network Administrator to ensure that the Firewall does not employ "port 25 blocking".3) You may need to change SMTP to port 587 please see http://www.pslhosting.com/cp/knowledgebase.php?action=displayarticle&catid=2&id=103 for info on how to do this