Emails missing from Sent Items

[Draw2Button]

For about a day I had a problem where Outlook wasn't automatically sending emails, they were sitting in my Outbox until I manually hit Send/Receive, and then they sent (or so I thought).

But looking for emails sent during that period they do not appear ANYWHERE in my Sent Items. I know they were sent because some of the recipients have confirmed receiving them, and my Inbox icon on the email has the tag for "You replied to this email on <x>", but if I search for all related messages nothing appears. I seem to have no record of these emails being sent.

I am using Exchange Online with Office/Microsoft 365 and Outlook on Windows 10.

Before you ask:

No, they are not deleted.

No, they do not appear when I try to "Recover Deleted Items"

Yes, they were sent.

[#2]

This has happened again. I've double-checked in OWA and the emails I've sent do not appear there either. This is maddening. Will someone please respond?

[#3]

Same issue in two different users, they also works in different domains and using different hardware. Any solution?

[#4]

@TonyPeperonniI ended up contacting support who believes it was related to a server-side issue. They ran some scripts and said the problem should be corrected. I haven't had it happen since but I'm keeping an eye on it.

[#5]

@Draw2Button Thanks for the info. I will contact support too, because I am the IT technician of the company and now there are 3 users with the same problem, so I need to know the solution in order to fix this issue in other users.

[#6]

@Draw2Button 

I just found an email was not visible in Sent Items. Luckily it was a Forward so I could find it by 'find related items' but still it was not visible, either in Outlook desktop or online.

The reason seems to be simply that I had added a tag (category) to it. As soon as I removed the tag it was visible again. But there is no filter set up in my view settings, so this just shouldn't happen. It has wasted time and shaken my confidence in the system.