When you send email, are you sending email to a server outside your network? or is it to a server thats internal on your network?
If its internal, then whoever set up your network probably blocked you from accessing the outside world. You'd probably have to speak to whoever your network admin is to fix that.
If your email server is external to your network, then you'll probably still have to speak to your network admin ... in this case they are probably blocking a specific port, port 80.
If this is at work, its possible they block port 80 to prevent people from spending time reading webpages instead of working.