I have tried to work this out on my own and read 4 million articles on the web buit can't get it yet and know there is a simple solution.
I have a global domain box email@example.com hosted for me and user accounts firstname.lastname@example.org also hosted.
I have all of this working fine but when someone sends an email to email@example.com(exchange hosted) and also an firstname.lastname@example.org (isp hosted) they get a ndr saying email@example.com failed. The user has collected the mail through their isp hosted email but the sender of the mail does not think they have as they have got an NDR for them.
This is only a problem if there is an internal and external username in the to field.
Any help appreciated.