Today while catching up with a friend at another company, (on a separate MS tenant) he brought up this exact scenario to ask if I had ever seen it before. To check, select the Tools tab, and move the slider to Online. If email continues to sit in your Outbox, you might be offline. Tip: If you don't see the sidebar, click View > Sidebar. The Outbox appears at the top of the sidebar only when messages are waiting to be sent. So, in this situation, you need to check email settings and check whether this issue is fixed. We figured this was a client side problem so we reimaged his laptop and will follow up in a few days. When you're online and email works normally, you won't see the Outbox. If the email is set to delay message sending, you will see the email in outbox for a while and you will come across the issue Outlook emails stuck in outbox. I checked the message trace, and can confirm that these messages he sent never made it to the server as the message trace shows nothing sent during that time window. I checked his outlook client for rules, and he has none.

The rest of the emails he sent shortly after this went through. I've dealt with him before, and he doesn't tend to make things up, so I'm not doubting that he in fact sent these (10) emails. no record of it in the sent items, its not in drafts, and its not stuck in the outbox.

I have a weird one, one of our users reported that a few times per week, he will send email that just vanishes.

Where is outbox in outlook 365