SMTP Error 73?

Greetings,

I have a user who gets a message delivery failure with the failure saying:

Delivery Failure Report

Your message:

XYZ

was not delivered to:

XYZ@Company.com

because:

73

I looked up what an error 73 is and it says:

-73 ERR_NO_PREDFINED_MX No Predefined mail exchanger

What does this exactly mean? Is this an issue on their end or mine?

Thanks