Fire can yoga

What does SMTP Reply Code "554 Transaction Failed" mean? - smtp 554 you are not me

A user at work can not receive e-mails from a particular sender with an e-mail account at Yahoo. The sender receives an error message like this:
"The SMTP server program
\\ \\ \\ \\ \\ \\ \\ \\ U0026lt e-mail: DNSname host (IP)
refused to talk to me: 554 Transaction Failed in denial-of-Login Control Listed.
We never see the e-mail in our mail server at work, so that he never for us.
I do not know what they think of the issuer to try to repeat it. Suggestions?