I administer my work's network and I encounter a problem I never got before.
I got a user (firstname.lastname@example.org) who sends email to on of our customers (email@example.com). When firstname.lastname@example.org sends an email to @customer.example.org domain's, the email gets duplicated by the number of people who are supposed to receive the email :
Example : - 2 people in the TO field : they both get 2 copies, - 1 person in the TO field and 3 in CC : everyone gets 4 copies.
At @customer.example.org they only have the problem with email sent from email@example.com. When firstname.lastname@example.org sends email to other domains, the problem doesn't happen. When another user from my company send an email to @customer.example.org the problem doesn't happen.
It might not be super clear, so if you have any questions, i'll try to answer as best as I can.
PS : I forgot to mention that we are using Outlook 2003.