530 validating sender
It can happen if the recipient doesn’t allow relay, then the sender will not be able to reach or that the receiver domain is restricting the sender domain from relaying the message.For Exchange 2003: If you have been using any CRM application such as Share Point or Dynamics and facing the above error: "550 5.7.1 Unable to relay".I tried all the different ports, 25, 80, 3535 and 465 with SSL. Go Daddy’s stellar support referred her to Micro$oft.And of course they wanted to charge her for support on a brand new Dell Inspiron 15.Simply put, a 550 error code means that your SMTP server isn't able to deliver the sent email to the user because his mailbox does not exist: either the customer has entered it wrong (a typical example is something like [email protected] of [email protected]), or the account has been deactivated and replaced with a new one (but you didn't know it).
It generally comes with a side-message like "550 Requested action not taken: mailbox unavailable" or "550 5 2 1 mail from refused spam site".
Just like 550 5.7.1 it also indicates that the user email client failed to authenticate on the mail server.