Large organisations who have not checked whether their email actually works. I emailed Radio NZ's technical people using the email address on the RNZ website.
It bounced saying that no such address exists...!
A local school updated the email address on their website. Unfortunately they only updated the text portion, not the URI (mailto: in this case) pointed to the old address that bounced back. When I pointed it out they said it must have been way for MONTHS. Whoops!