I can recieve emails, but I cannot send any.

[bella]

The error message that I get is:

Subject 'Re: National History Day', Account: 'mail.optonline.net', Server:
'mail.optonline.net', Protocol: SMTP, Port: 25, Secure(SSL): No, Error
Number: 0x800CCC15

Also, I can send emails through the webmail.

 

[Gary VanderMolen]

There seems to be a leading sentence missing from that error message,
possibly something like "A TCP/IP error occurred while trying to connect
to the server. " Please don't leave out any parts of the error message.

That type of problem is usually caused by interference from a non-
compatible antivirus program. Which antivirus are you running?
As a minimum, email scanning in the antivirus must be turned off,
although that may not be sufficient to eliminate all bad effects.
In a worst case scenario, your antivirus may need to be uninstalled.

Email scanning in any antivirus must be disabled, for reasons
explained here:
http://thundercloud.net/infoave/tutorials/email-scanning/index.htm

If no improvement after disabling email scanning, consider upgrading
to Windows Live Mail, because it is less prone to suffering bad effects
from overly intrusive antivirus programs:
http://get.live.com/wlmail/overview