Emails going to a wrong mailbox

Any forwarding rules from the mailbox she is receiving them from?

no forwarding rules.

So now, it seems like he was getting only one persons emails.

Look at the persons email she was receiving, see if there are on OOO's (Out Of Office) on it or anything set that could have been accidentally tuned on for that time.

Unless this is set in the EMC to forward emails, the only other way I know is that a rule or OOO was set.

What do you mean by receiving other user's emails?
Mails addressed to others? May be he was in a group to which the mail was sent!...

What does message tracking tell you? Should be able to determine how the email was routed to the CEO or the user that sent to the CEO, tack this as well.

The user who's email was sent had the OOO set with incorrect dates.  That seems to be the timeline when the emails started to go in to CEO's mailbox.  When the OOO was corrected again that's when it stopped.

Wouldn't the ooo identify itself as out of office, how was it confused and miunderstood an email was supposedly coming from another user? Just curious.

It was the OOO sending the email. I believe, OOO will only send one notification of the person being OOO during the span of the OOO. I don't remember if this is true or if the user has the option to set it. It sounds like the user inadvertently had them forwarded via the OOO.

Please elaborate your solution.

I got that.
As for the solution...to err is human then.

It was not a technical issue.