[exim] Mail to literal "*@mydomain.com"

Highly unexpected today, every user on my system got an email
when mail was sent to exim addressed to (literally)
"*@mydomain.com". This was unexpected behaviour, to say the
least. We use both a traditional aliases file and an LDAP
lookup to AD, though I don't think that either was used in
this instance as the headers on the message were:

Envelope-to: *@mydomain.com
To: *@mydomain.com

Any idea how this happened (I presume it doesn't work on all
exim installations - try it!)? And how can I stop it?

exim version 4.6

-- 
Illtud Daniel                                 illtud.daniel@???
Prif Swyddog Technoleg                          Chief Technical Officer
Llyfrgell Genedlaethol Cymru                  National Library of Wales