I assume you get this in your e-mail program...always helps to know these
facts.
If that is the case, you first need to be sure your e-mail setup is correct,
for that you need the instructions from your ISP. If setup is correct, then
you might turn off your firewall and see if you get the error. If yes, the
problem is in the firewall and you need to find the settings in your
firewall to allow the e-mail app to send. If not, you may have a setting in
your antivirus software that is preventing the send so you'll need to check
that as well.