Error 554 – Access Denied

I have received a failure message in response to an e-mail I sent, with the following message:

Hi. This is the qmail-send program at yahoo.com.
I’m afraid I wasn’t able to deliver your message to the following addresses.
This is a permanent error; I’ve given up. Sorry it didn’t work out.

[address of destination company]
Connected to [IP number] but sender was rejected.
Remote host said: 554 5.7.1 : Sender address rejected: Access denied

Any idea why?