More and more ISPs are requiring their customers to use their outgoing mail server instead of your domain's SMTP server. This is an attempt to fight against spam, which has been increasing. Hopefully if your ISP adopts this policy you'll be notified, but that isn't always the case. So if your email suddenly stops sending it's possible they've blocked the use of any SMTP server other than their own. This is normally port 25. You can try changing the outgoing port in your email client ( Outlook, Outlook Express, Eudora..etc ) to port 225 to see is that helps, but if not you'll need to change your settings to use your ISP's SMTP server. A link to instructions on how to change these settings in Outlook 2003 and Outlook Express is below.
If you experience problems sending email, please contact your ISP to determine if it requires different SMTP server settings and what those are.
Configuring Outlook 2003 and Outlook Express to use our ISP's SMTP server
Here's a partial list of ISPs that require you use their SMTP server:
MSN
BellSouth
Verizon
Earthlink
Comcast
