Service Providers have moved toward restricting access to their outgoing mail servers to provide better service to their customers and prevent SPAM from being sent through their mail servers. There are different methods for establishing restrictions that could result in you being denied access to your outgoing mail server.

When you connect to a mail server and send mail that is going to another mail server on another domain, that is called mail relay. Historically, SMTP servers did not check to verify that the sender was who he claimed to be and would simply pass the mail on with whatever return address was specified, no questions asked. Unsolicited bulk mailers have taken advantage of this to send huge volumes of mail with bogus return addresses (which slows down the server for the paying customers who have the right to use it, slows down servers everywhere with all that junk mail, and makes people mad at the ISP for allowing the bulk mail to cause such problems to be sent through their outgoing server).

One of the ways to restrict access to an outgoing mail server is to verify that the computer is on the ISP's local network. When you dial your modem and connect to your ISP, your computer is given an IP address that identifies you as being a part of that ISP's network. If you have two ISPs and dial up to one and then connect to the other's mail server, it may prevent you from relaying mail because your computer is not identified as being on the local network for the provider whose mail server you are sending through. In this case, you should try to use the SMTP server for the provider you have used to dial up and connect to the Internet.

Another way to restrict access is to insist on a local domain return address. If you connect to the mail server for "domain.com" it may only allow you to send mail that is from "username@domain.com" -- if you try to send from another account and have the return address of "username@anotherdomain.com" it may restrict you from relaying to another server.