sending email address changes after going out

When I send an email, it goes out as XXX@YYY.com, but is recieved by the recipient as XXX@YYY.comcastbiz.net, and if they hit reply it then sends thier response back to the @YYY.comcastbiz.net address. 

Any ideas why this is happening would be appreciated. Comcast support can't figure it out.

Thanks