exchange server acts selectively

I run a computer network in a large newpaper/publishing house,we get e mails from reporters and global correspondents.My exchange server acts weird,some of the e mails are recieved and some are not.Ofcourse I am pretty aware of my block list and spam filters,in fact all the not-recieved emails come from addreses that are on the client’s contact list and their messages are regularly recieved no problem.Seems that the mail serve has a mind of its own and decided one day to block those addresses then allow them after a couple of days.no intervention was made by the mail admin.
any inputs on this topic?