Email received multiple times for one domain depending on number of recipients

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http://serverfault.com – I administer my work's network and I encounter a problem I never got before. I got a user ([email protected]) who sends email to on of our customers ([email protected]). When [email protected] 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 protected] When [email protected] sends email t (HowTos)