The help pages offer a solution though, use the plugin called Postie.
I installed Postie. First problem was that to benefit from all its features I had to install php-imap. Thats a aptitude install php5-imap command if you have that sort of access to your apache (Debian) server.
Once I had fill in the configuration page, it was time for testing. I sent an Email, and clicked the manual test button. It failed – An unauthorised Email address. As part of the configuration, you can set up a list of valid Email contributors. However, my list of contributors included the unauthorised address.
Even after cut and pasting, and making sure I had a carriage return at the end of the address, with no extraneous spaces, I was still getting the Email rejected. At this point, I selected the option “Allow all Emails to post”. Again, the post failed due to an unauthorised Email address.
It was then I saw the problem. Postie had a default setting for poster user name of “admin”. My administrator account had a different name, admin did not exist. I changed the name to my administrator and it worked.
In conclusion, make sure Postie has a valid user name for posting, otherwise you will be getting a misleading error message.