If they were using ActiveSync, their devices wouldnt be accessing the SMTP service.
Also, you stated before "whenever any users iphone tries to get email." SMTP isnt used for getting mail, so something else is going on not related to the Iphones.
Is 64.168.166.241 a client you are responsible for?
Does the Exchange server accept mail directly from the Internet?
http://support.microsoft.com/kb/843106How to troubleshoot the "504 need to authenticate first" SMTP protocol error