An incoming external email was not received by all recipients

Are you using the built in Antispam agent in Exchange?
Do you have an external spam filter that processes all incoming emails before being delivered to your exchange server? If you do I would look at those logs to see if the email arrived at your premises.
A few things you could check.

Have fixed one yesterday similar.



                  - Recreate profile name of Mail account
                  - Modify ports of ISP in/send as SSL special for office 365 outlook
                  - Del then recreate new outlook account of Sync Activate


                  #  When sporadic met an envelope on the icon of outlook , whatever click the sync , update , junk and receive all , there is no any more one email added , consider the exchange server , to one of your Lan cabling outlooks of PCs being blocked by , a outside program or inserted in your computer .

If you have content filtering for email on your firewall it may have been blocked there as well. I dealt with watchguard devices that had this enable and it would completely delete the email and the email was non recoverable!!!! Not sure why they designed it that way!!! No quarantine - really people!

If one of the three received it, it likely isn't spam filtering as that should have blocked it for all three. Normally, in these circumstances, what I commonly see is that the users who did not receive it have an Outlook rule that acted on the message. You do say that the tracking logs show the mail was not delivered to these two though. The other possibility is that the sender mistyped the email addresses of the other two recipients. Have you taken a close look at the message to see if that was the case?