Two of the more common causes for this are interference from an
antivirus program, and setting the outpoing port number to the wrong
value.

What if any antivirus programs do you have, and have you ever had
a Norton or McAfee antivirus program installed on that computer?
New computers often come with a trial version of an antivirus program
already installed.

What email provider are you trying to use?  Many people choose
their internet provider as their email provider also, since most
internet accounts include one or more email accounts.

If you have the outgoing port number set to 25, is your email
provider the same as your internet provider?  Connections to
port 25 that cross over from one internet provider's equipment
to another's are often blocked  to cause problems for spammers.

Also, have asked your email provider is they have problems with
their outgoing mail server?