A: When you open Microsoft Outlook offline and compose and send an email message, the outbound message sits in the Outbox until connectivity with the configured SMTP server for the profile is available. The same thing happens when the SMTP server is unavailable for whatever reason.
At home, I have multiple wireless access points. One is a residential cable modem, and the other is a business account. For the residential account, all SMTP email must be sent using my cable provider's SMTP servers. With the business account, I use my SMTP server.
My Windows laptop is configured with wireless access to both of those Internet access types. While working, connected to the business account, I experienced an interruption of some kind that blocked Internet access on that account, and my Windows workstation switched to the next available, configured wireless account. This way, Internet connectivity was maintained.
I didn't notice the change in wireless access. Outlook, on the other hand, was now configured for my mail server but had access only to my cable provider's mail server. I sent some messages using Outlook in this state. As expected, they made it to the Outbox. However, I wasn't attentive enough to notice they didn't leave. My laptop is on all the time. It took a few days before I realized messages didn't get sent.
The only lesson I can really derive from this experience is either to not let Windows switch freely between Internet connections with different messaging configuration requirements, or to watch the Outlook Outbox to make sure important messages you send actually make it safely from the Outbox. Outlook also shows a small error on the status bar at the bottom of the screen when Send/Receive is completed if its unable to send messages. Pay attention to it.