Mail delivery with same first names

How do you tell Notes not to deliver a message to a user unless the ‘To’ address matches the users email address as specified in the person doc?

We have a number of Dave’s and David’s on our network. The email address format we use is firstname.lastname@domain.com but if we get an email in for just say ‘dave@domain.com.au’ it gets delivered to one of the Dave users anyway even though the address specified in the person doc is dave.lastname@domain.com

Its seems that Notes delivers it to the first ‘Dave’ it finds in the directory no matter what email address is specified for that user?