Issue with emails being delayed when sent

Good Morning, 

I am working with a user who is sending emails, but they are being delivered a week or in one case two weeks later.  She is not using Delayed Send\Delivery, that was the first thing I asked, or at least she said she didn't. This is not all the time and we were not able to replicate while I was on a call with her. 

What she is doing...

Open a document in MS Word to fill out a report and Save it. 

Share the document, attach a copy instead and choose PDF. 

This opens, the email window, add the recipients and hits send
Let's say for example she does this a 2pm on 12/4 

Her sent items in Webmail show it was sent from her mailbox at this time. 
Recipients might receive it a week later on 12/11 at 11am Message traces in Exchange Online console show the delivered on this date, and her sent items in the Outlook Classic version on the desktop show the 12/11 date as well. 

My question would be, besides Delayed send, any thoughts as to what might cause emails to sit undelivered and then just be released a week later?  I even checked our backup software and looked at the backup for her mailbox during the timeframe of it being sent vs. delivered to see if there was something in her Drafts or Outbox, but I did not see anything there.  She says she did not do delayed send and I can't prove she didn't. 

In the interim, I did suggest she put a delivery receipt on these emails since they are time sensitive and need to be delivered weekly, so if she does send it and does not get the Delivered receipt, we can take a look right away.

Thoughts or suggestions...

Thank You