I came here to post about the same problem.. I've checked the intended recipient's outlook for rules, but there are none setup.. That's as far as my knowledge extends, so if there is other things to check, please let me know.. I have server access in case it is a configuration on the server that needs to be checked.. Also, ill write out the problem:

My coworker is receiving emails sent to other employees..

her email is janedoe@company

she receives emails that are meant for johnsmith@company

johnsmith also receives the emails meant for him

when jane opens the problematic emails, they say to: johnsmith@company

janedoe also has the same problems with receiving emails that are meant for frank@company

Oddly, both frank and john were laid off for about six months, so there may be a forwarding setup that was never removed when they came back to work that may of been setup by the old IT company.. but i've check both their computers and neither have any rules set in place..

so right now, if i emailed johnsmith@company, he would receive the email, and she would also receive the email.. I assume the problem is with the server options..