I have one user with office 2007 that has created a small dis list that contains 7 members and after she sends an email to it, she look at her sent item and once in a while a name is not there (and of course that person did not receive that).
Some names are internal employee in our exchange organization and others are external contact created in her outlook contacts.
The problem affect both types (internal and external contacts), and it's not always the same names and most of the time they work.
In my exchange logs, I see that the name that is missing in her "sent item" email was also not process by exchange so it looks like a local outlook issue.
Any idea of a solution?