Since the last months, the number of viruses propagating by mail and running their own SMTP engine has been dramatically increasing. Those viruses work in a simple way: when a computer is infected, the virus scans files on the disk drive, searching for email addresses, and sending itself to each collected email address, using another collected address as sender.
That kind of virus attack is creating huge performance problems on the network, and for this reason many ISPs have closed the SMTP port (25) for outgoing connections from their customers, so an infected computer cannot propagate viruses to any external email address by itself.
The unfortunate side effect of this measure is that legitimate customers cannot connect anymore to external SMTP servers, and particularly to CERN SMTP servers, to send mails.
Check and see if your ISP has shut down access via this port because it detected problems with your address.
"Nowadays, many Internet providers are closing their SMTP servers and filtering all the incoming SMTP traffic to fight Spam. They let everybody in from their local network. And filter everybody out who is not. This means that as long as you are using their services, your email client is working fine, but what happens when you go on a business trip or vacation and you want to send an email from an airport or hotel room? In most cases your SMTP server is not accessible to you anymore since you are trying to get to it from outside of your ISP's network."
Some ISP's prevent their users from accessing "outside" SMTP servers using conventional e-mail programs like Outlook, Outlook Express, Eudora, Netscape. The error message will look like:
"Unable to connect to the server. (Account:'account', SMTP Server: 'mail.YOUR_DOMAIN.com', Error Number: '0x800ccc0e')."
To verify that your ISP closed SMTP connections on port 25 you need to "telnet" to SMTP server from the Command prompt (DOS prompt):
telnet mail.YOUR_DOMAIN 25
You will either receive SMTP server prompt, or "connection refused" message, or timeout.
In the case you receive "connection refused" message or timeout, you need to contact your ISP with the request to open port 25 in their firewall or proxy. Or, try using their SMTP server for your outbound mail connection.