Not sure how long this has been happening but i got told this morning that a user in the domain is sending emails to other domain users (within the company) and some of those emails are being received by another domain user but they are not included in the addressing nor do they show in the header details..

How can a user receive an email when they are not listed in the To, CC or BCC?

I checked the original sender who uses Outlook and they have no rules setup to BCC

UPDATE
I found this entry in the To field on one of the older emails sent over a week ago:-

/o=domain.com/ou=SmarterMail/cn=Recipients/cn=b3e76c27b85a438ba64bae60095e 5193-Username </o=domain.com/ou=smartermail/cn=recipients/cn=b3e76c27b85a438ba64bae60095e5193-Username>

I took out the original domain and username for privacy/security.