Just sharing something sightly different but weird. It could happen to anyone.
I received an email in my Hotmail. Opened it up in my Outlook for Mobile on my Android. Quickly replied and hit send. Apparently Wifi was very poor yet mobile data was on. I refreshed Outlook many times. However the email did not go or appear as sent. It was not appearing in any other folders either. I quickly went to the PC and opened up eM Client. I refreshed several times. Only the incoming email appeared in the inbox. No email appeared in Sent items and no outgoing email stuck in outbox either.
However weirdly the email appeared as half composed in the sent folder on eM Client on the PC. I then deleted that half composed email from the sent folder that appeared as sent to the recipients. I then quickly disconnected the phone from mobile data and WiFi. I even uninstalled Mobile Outlook on my phone to prevent it from sending out the missing email reply. I then composed another reply and sent the email from the PC. This email was sent successfully and appeared in the sent folder in eM Client.
Shortly afterwards, the full original email from the Outlook Mobile that had disappeared mysteriously also appeared in the sent folder a minute later in eM Client.
It was very embarrassing to have Outlook/Hotmail send out two email replies with slightly different messages to the recipients. Outlook Mobile does not even have an Outbox.