This error usually occurs when the recipient or one or more of the recipients of your e-mail mail message has an invalid e-mail address.

For example, if you forget to put an ampersand "@" in the e-mail address, the SMTP server will automatically reject this and will not send the e-mail message. You will need to go through the recipient or recipients of your e-mail message to make sure the syntax (username@yourdomain.com) is correct.