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.