users complaining that their not getting all their inbound e-mails

I am using exchange 2003 and my users are complaining that they are only getting some of their incoming e-mails but when I test them to see who is getting what and who is not I can get a response
! note
not all users are using office 2003
some are using office xp and some are using office 2000
any suggestions would be greatly appreciated
Thanks
CClouston