From
java FAQ:
Q: When I try to send a message, why do I get javax.mail.SendFailedException: 550 Unable to relay for my-address?
A: This is not a JavaMail problem. This is an error reply from your SMTP mail server. It indicates that your mail server is not configured to allow you to send mail through it. Typically, mail servers for an organization will be configured to allow mail from within the organization to be sent to other addresses within the organization, or to addresses external to the organization. It will also typically allow mail coming from an address external to an organization to be sent to addresses within the orgnaization. What it will typically not allow is mail coming from an address external to the organization to be sent (relayed) to another address also external to the organization. The configuration of the mail server determines whether such relaying is allowed, and which addresses are considered internal vs. external. Often mail servers will require you to authenticate before they will relay messages
