Exchange 2013 - Missing Emails sent internally

Question

Can someone please help me diagnose what has happened with the following problem
User1 sends an email at 23:18 from his works laptop at home which is connecting to exchange using Outlook Anywhere
User1 sends an email to 4 users: "User2, User3, User4, UserEx" - UserEX is an external user.
User3, User4 & UserEX - Received the email
User2 - Did not received the email
User1 - Travels to place of works and logs onto his works PC (not the laptop which is at home)
User1 - Also receives a copy of all messages sent to User2 and notices that he doesn't have a copy of the email he sent to User2 at 23:18.
User1 - Checks his sent items, there is no email there from 23:18
User1 - Confirms that User3, User4 received the email, which they have.
He contacts me (support) asking what is going on. I investigate by looking at his OWA (thinking it could be a syncing issue), the email doesn't show up UNLESS I do an advance search on his OWA Email Box, which tells me there are two emails 1 in his Inbox
and 1 in his drafts. Move away from the advanced search and the email cannot be found.
I complete a Deliver Report for User2 for this missing email; Results below
Submitted
19/01/2015 23:18 EXC01
The message was submitted to exc01.*.
Pending
19/01/2015 23:18 exc01.
Message was received by exc01. from EXC01.
19/01/2015 23:18 exc01.
This message was redirected to another email address.
19/01/2015 23:18 exc01.
The message has been transferred from exc01. to EXC01.
Delivered
19/01/2015 23:18 exc01.
The message was successfully delivered.
Even the Get-MessageTrackingLog tells me the message was sent. Though there was a HAREDIRECTFAIL & AGENTINFO in the processing.
If anyone can help me workout what the heck is happening here, id be very appreciative.
Thanks
Neil

Answer 1

Hi there Amy,
User1 sends about 10-20 emails a day to User2 and they have been fine since and beforehand. Its just this instance.
User1 = Director
User2 = PA
It was an Email not a Meeting Request.
The email in question was being replied to.
To: UserEX
Cc: User2; User3; User4
The Director receives a copy of all emails sent to his PA. In this instance neither one of them received the email. The only people to receive the email was UserEX, User3, User4.
Also the Directors email (User1) wasn't in his sent items on his work PC in the morning. (Remember he sent it from his laptop at home during the late evening the day before).
The trackinglogs & delivery reports, say that the email got delivered to all recipients. Further investigation suggests that this email is there but not visible to them.
Its a strange one I know, but I'm wondering what would cause an email to be there but not invisible?.