Microsoft Exchnage Server filing to recognize email addresses in my contact list.

I sent out an email to tow of my co-workers.  Bothe emails are correct; been using them for years.  I recived an email with the two coworkers names but the email address was changed to IMCEAEX-N@w14c.comcast.net  The error code is 

#550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ##  The generating server is W14HUB-WC-C6.w14c.comcast.net.  I have checked the online echange account with the desktop account and all setting are the same.