There are only two reasons that E-mails fail to deliver and essentially they are the same.
The address is not recognised by the server.
1. The address you sent out is incorrect
2. The address you sent to doesnot/no longer exists/is inoperative.
Carefully check the Address book entry you have
it takes but a dot in the wrong place, a capital letter, a ingle character not as it should be.
This could be because of a copying error when you obtained the address by incorrectly writing down when given or even by the giver.
The best way to sort this out is to compose a message then send it to a series of addresses based on variation of the original- in the title line give each address version a different I/D 1 2 3 etc. When they bounce back make a note and delete that address. The one that does not come back is the one to keep so to speak.