Sometimes when I try to send mail I get the error “Some of the recipients were rejected by your mail server”. Why do I get this error message?

This error occurs when the e-mail address you are trying to send to does not exist. If you replied to an e-mail and received this message, this means that their “reply to address” is wrong in their mail program configuration. You will have to manually enter their e-mail address if you know it in order to send the message. You may want to inform them that their reply to address is incorrect.