Delivery Failure Message

Comcast hosts our domain, we are not able to send an email to one of our clients.  The error message is:

The recipient's e-mail address was not found in the recipient's e-mail system. Microsoft Exchange will not try to redeliver this message for you. Please check the e-mail address and try resending this message, or provide the following diagnostic text to your system administrator.

The client states that this is an issue with our email service.  The email address is a valid one.  Do any of you know how we might be able to correct this?  Thanks!