Errors 550 & 553

SMTP Relay Blocking Error Message When You Send Messages

When you send messages you, may receive an error message that your message cannot be sent because of Simple Mail Transfer Protocol (SMTP) relay blocking. It will be similar to the following error message:
The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'nobody@gct21.net'. Subject: '', Account: '', Server: 'smtp.gct21.net', Protocol: SMTP, Server Response: '550 nobody@gct21.net... Relaying Denied', Port: 25, Secure (SSL): No, Server Error: 550, Error Number: 0x800CCC79
OR - the message may say this- *553 sorry, that domain isn't in my list of allowed rcpthosts {#5.7.1}