Most ISPs, including JTS, restrict access to their outgoing mail servers to prevent spam from being sent through their mail servers. If you are getting the "550 Relay Denied" error message, the outgoing mail server cannot verify who you are and will not allow you to send mail.
Mail Relay is when you connect to a mail server on another domain and try to send mail (for example, if you are connected to the Internet through AT&T and then try to send mail using Earthlink's mail server). Historically, mail relay was fine—SMTP servers did not check to see who was sending the mail and would simply pass the mail on, no questions asked. But unsolicited bulk mailers (spammers) have taken advantage of this to send huge volumes of mail with fraudulent return addresses (so as not to be traced), which slows down the server for the paying customers who have the right to use it. This also slows down servers everywhere with all that junk mail.
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.
For example, if you connect to the Internet with AT&T and try to send mail using Earthlink's mail server, you will get a 550 error. To fix this, 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 from "firstname.lastname@example.org." If you try to send from another account and have the return address of "email@example.com" it may restrict you from relaying to another server.