Its weird that you can receive mail but can't send. Most mail servers don't require you to authenicate on a send just on a receive to verify your account credentials. They know your IP address as they give it to you when you connect so they open it up to that block of IP's.
I'd wait till tomorrow or so. Chances are if you didn't make any changes to, install any software firewalls or other then its an ISP issue, especially if you have full internet access and that you can receive e-mail. Your ISP may be doing maintenance or feeding the hamsters who turn the wheels that give power to the mail server!
I've seen a problem on one workstation where after VPN'ing into their work they needed to reboot the PC to gain access to their private optonline e-mail again. They, however, couldn't send or receive after disconnecting from their work VPN. Again..sometimes a reboot of the PC and the modem fixes many internet gremlins.
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