I am using the TIdSMTP but when I try to send mail I get an error message such as: "521 potserver.organization.com access denied" or "'572 Relay Not Authorized."

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It means that you have been denied access to that SMTP server by its administrator.  Administrators prevent abuses of their SMTP relay servers by requiring:

 

That you send E-Mail only from a computer connected to their network.
That you use some type of authentication.

 

Talk to the server's administrator and ask what are the requirements for connecting to the server.  Sometimes, the server may need to be reconfigured.