very common bounce back message below
Delivery has failed to these recipients or distribution lists:
Microsoft Exchange has been trying to deliver this message without success and has stopped trying. Please try sending this message again, or provide the following diagnostic text to your system administrator.
then Delete the auto complete entries. By typing the first few letters of the email address using the arrow keys until it is highlighted then press the DEL key. You can now retype the address manually and try again.
hope that helps, I've come across this many times I can't recall the specific reason for this issue but deleting the auto complete entry and retyping the address manually has worked for me, hope it works for you