554 is a pretty generic error code for a failed transaction (if that is indeed an error code, but it's the only thing that resembles one in that message). These days it's mostly used to indicate a blockage due to spam filtering or policies.

You really should check with your mail provider to see if they can pull some more details from their outbound server logs. You might also want to check with the recipient to see if they have your email address blacklisted for some reason. It's unlikely to be a server or domain blacklist, as you said other people in the office can send him email.