I'm using version 18.104.22.168 using an Office 365 account installed as a snap on Ubuntu 17.10.
Something happened that stopped messages from going out. Instead they are stuck in the Outbox folder. Logs don't show any obvious errors. Some sort of error should be generated in this circumstance - either in logs or in the GUI. There should also be a way to force a retry for these messages. If retries are happening in the background, show some sort of feedback to the user.
It's also worth mentioning that I can't send feedback in the client because it just gets stuck in the Outbox.
Until I can free messages from the Outbox, I'm forced to fall back to another email client.