We are facing an issue where an email sent to a single recipient within the organisation, gets copied to the sender's inbox as well as gets cc'ed to others in the organisation. 

For example, person1@organisation.com sends an email to person2@organisation.com

person1@organisation.com receives that same email in her INBOX as an email

person2@organisation.com receives the mail as expected.

person3@organisation.com also receives a copy of the same email although person3@organisation.com was not on the To, or CC or BCC addresses.

Please help us understand this problem and steps to resolved it. 

Thank you!