This problem does not effect users that are using WebMail to access their email. It occurs to customers of few ISPs which are also doing this, including: MSN, Optimum Online, Charter and few others. Your ISP could've just started doing this recently.
Customers will not be able to use their outgoing SMTP mail servers (mail.yourdomain.com) that are provided to them with their hosting account. ISP blocks all outgoing email if you aren't using their own SMTP servers. If you are using your own SMTP servers you will see the following error or an error close to this one:
The connection to the server has failed. Account: 'youraccount', Server: 'www.yourdomain.com', Protocol: SMTP, Port: 25, Secure(SSL): No, Socket Error: 10060, Error Number: 0x800CCC0E
You will have to use your ISP's outgoing mail servers for all of your email accounts. Your email will still be from your own domain name, the only difference is in the way it is routed throughout the Internet. Instead of being sent through our servers it is processed through your ISP's servers. This allows your ISP to monitor its users and to make sure they are not sending out SPAM. To set up your ISP's SMTP servers for your outgoing email you must click on your account settings in the email client you are using (Outlook Express, Eudora, Netscape etc.) In the Outgoing Mail Server (SMTP) field you will enter your ISP SMTP server as your SMTP server. You must then enter your ISP's username and password for that mail server. In Outlook Express it is at the bottom of that same tab. You will check off "My server requires authentication" and you'll click on "settings" to enter your ISP's username and password.
This problem came to my attention because of few customers having problems with sending emails. This solution should work for most.
If you donnot know your ISP's smtp server, please contact them.
Alternative(Used port 465 for Secure SMTP)
In Outlook , in the E-Mail account screens select the account you wish to change, set the SMTP server to your own domain and click More Settings. On the Advaced tab check the box under SMTP that says "This server requires an encrypted connection (SSL)" and change the Outgoing server port number from 25 to 465.