However when I send them in I get a mail in my inbox with the header "Mail Delivery failed" returning mail to sender.
Message in mail as follows;
There are more, some of it contains info on the return path, but about 90% of the message seems to be gibberish or code.This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
<Error details edited out>
I can post the rest, excluding my mail, if that is of any help.