mail being forwarded and server-side BCC'ed without a trace

We have a user's mailbox (Office 365/hosted Exchange) that was copying all messages to an external (unsolicited) recipient but not leaving a trace in the mailbox. I have removed two addresses in mail forwarding and inbox rules, but I am unable to find where the outbound messages are being sent and removed from the sent folder.

Steps taken:
  • Forced password reset on account
  • ran malware/antivirus - (no hits)
  • checked rules in Outlook and OWA - (found rule in OWA that was forwarding mail to a different malicious address)
  • checked Outlook for RSS feeds - (none listed)
  • checked server-side transport rules for tenant- (nothing applicable)
  • checked mail forwarding in OWA - (found the same email that was receiving all sent messages)
  • checked user's Outlook for VBA scripts - (blank)

What other place should I be looking?
Johnathon HarrellManager of Information TechnologyAsked:
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timgreen7077Exchange EngineerCommented:
I would actually blow away the mailbox and email address just to be safe. also this is most likely something on the user's computer so I would wipe the computer or reissue the user a different computer and wipe the old one. if you blow away the mailbox be sure to export the mail first.
"I am unable to find where the outbound messages are being sent and removed from the sent folder."

It won't be in Outlook Sent box if each incoming email was forwarded on the Exchange side (delivery options > forward to).  This feature will be under mail flow in newer Exchange 2013/2016.  If this is the case there should be an entry under Contacts to the external email address.

Did the person have admin access to Exchange?

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Johnathon HarrellManager of Information TechnologyAuthor Commented:
I was getting hung up on the fact that valid sent mail was being intercepted. It turns out a previous administrator created a transport rule for just this user to CC himself on every message. This meant that all sent messages returned to the mailbox and then were forwarded on to the malicious email.
Glad you were able to solve it and thanks for getting back. Cheers!
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