553 mail from must equal authorized user

A cryptic message!

After some research, I think I have found the answer:

553 is a SMTP error code, which stands for "Requested action not taken: mailbox unavailable". This error code is often returned by a mail server when a sender tries to send an email to a recipient whose mailbox is not available or does not exist.

"Must equal authorized user" is likely a reference to the fact that the sender's email account must be authorized to send emails on behalf of the recipient's email account. In other words, the sender's email account must be a valid and authorized user of the recipient's email account in order to send an email to that account.

So, in summary, the message "553 mail from must equal authorized user" is likely an error message returned by a mail server when a sender tries to send an email to a recipient whose mailbox is not available or does not exist, and the sender's email account is not authorized to send emails on behalf of the recipient's email account.