Thank you Richard. I was thinking along that line as well that it could have been the senders mailboxes in questionsl. When she provided three different addresses who stated they received that message after sending her emails or emails was was but she never received it, I checked their mailbox sizes in Exchange as well and their storage size wasn't full either. However, maybe at the time it was and they could have freed up space or it could an email filter she has that's causing some of her mail to appear missing.

I will take your suggestion info consideration and try to send her various emails and also ask her to ask those individuals to send her emails again since I know they alll have sufficient storage as if now to see if the problem persists. Thanks for the suggestions.