Here is a excerpt from an article that I just read that could help with your issue.
ISPs block some outgoing e-mail unexpectedly
By Woody Leonhard
Recently, many Windows Secrets readers — me among them — discovered that they could no longer send e-mail, although they could still receive messages.
In an attempt to reduce spam, many ISPs, including Verizon as of a few months ago, now block all outbound traffic on what used to be the de facto avenue for e-mail, port 25 — leaving customers in the lurch.
E-mail glitches rate among the most difficult, distressing, and dire problems in all of computer-dumb. Orphaned e-mail programs, operating systems with more patches than a clown's coat, the whims of intransigent e-mail and Internet service providers, and the phases of the moon combine to make e-mail problems devilishly difficult to solve.
And any e-mail glitches you fix today will undoubtedly require even more remedial attention in a month or a year.
One problem pops up regularly over the years because an ISP suddenly blocks all outbound communication using port 25. This glitch has a very specific symptom: your e-mail program — whether Outlook, Windows Live Mail, Outlook Express, Windows Mail, Thunderbird, or Eudora — suddenly loses its ability to send mail. You can receive messages with no problem, but every e-mail you try to send sticks in your outbox.
In fact, there are about 10 million different reasons why your outbound mail may refuse to budge. Also, a very slow Internet connection may leave you thinking that your e-mail app has given up the ghost, whereas the mail is actually just taking its sweet time.
The port 25 problem is more obvious: messages get stuck in the outbox and won't transmit, no matter how long you wait. Surprisingly, Windows and your e-mail program probably aren't to blame. In this case, your Internet service provider has erected an enormous roadblock on your stretch of the information highway, and you can't get around it without a detour.