Using New Outlook on Windows for personal email, calendar, and contact management
I know what you mean. I have had this a few times and need to go back into old outlook to give it enough time to send. It seems to close the outlook window as soon as it is in the outbox but closes too early to send.
Horrendous system, trying to make people use an online version of outlook as their desktop app. Just go back to original and make it work properly!