-- Make sure your internet is connected. If you're able to get to different web pages, and receive mail, you're internet connection is fine.
-- You may be sending an extermely large attachment. Try sending a message without an attachment, and see if that works.
-- A firewall or anti-virus program has blocked your outgoing email. If you've recently upgraded your Internet Security software, or it was updated for you automatically, it may have blocked outgoing email. If you have any Internet Security or Firewall software on your computer, try disabling it and sending mail again.
-- Your connectivity provider is blocking outgoing (port 25) mail to the Internet. The cable companies are becoming more aggressive about this, since a huge volume of spam is sent out through their servers from computers with trojans or viruses. There are a couple of things you can do here, you can contact your connectivity provider and ask them, or set your email program to use port 587. You can use the help feature of your software client to find out how to change outgoing mail ports for your particular email client or search our knowlege base for 587.
-- There is a problem with your email program. This is the least likely, but it does happen. If you haven't already, reboot your computer, and see if that helps.
If you're still having problems, please fill out a trouble ticket and provide as much information as you can.