Mark, from research I've done this morning in other forums it seems there is a general agreement that
DomainKeys needs to be set up in each account's DNS Zone entry on my
dedicated server.
I began by finding this posting "
Getting your IP whitelisted with Yahoo (solving the deferred 451 problem)" in the SmarterTools forum — I attempted to post it here but failed as it's over 10,000 characters!
Did your team install DomainKeys in any of their experimentations?