Email messages not sending and stuck in the Outbox

This may happen if the Outlook client turned itself to "Work Offline" mode.  The following website from Microsoft explains where that setting is and how to turn it back on.

https://support.office.com/en-us/article/switch-from-working-offline-to-online-2460e4a8-16c7-47fc-b204-b1549275aac9