[Exim] LDAP query failures

I'm seeing messages of the type -


"ldap://localhost/?ispMailbox?sub?(mail=mary@???;bruno@anotherdomai
n.com;tract@???)" gave DEFER: ldap search failed: ()

in the exim panic log. These messages are obviously failing because they
are invalid requests as the request should be a lookup for a single mailbox
not a list. How is this sort of query appearing as exim will reject this
sort of input to a RCPT TO command.

Any pointers much appreciated.
Jeff.