I'm having several messages in my Outbox waiting to get sent out.
I noticed that some of these messages have a date and are written in italics. Other messages have no date and are in Roman. Upon sending and receiving, the ones in italics get sent but the ones in Roman hang in the Outbox.
What is the difference and how can I fix this without needing to recreate the messages?
If the message is not in italics and doesn't hold a date, it is not ready for submission and will hang in your Outbox until it gets resubmitted again.
This hanging issue often happens when the message gets opened (via a double click) from the Outbox, for instance, in order to modify something. If you do, do not use the Close button when done modifying the message, but make sure to press the Send button again to put it back in the queue.
Another visual difference is the icon of the message itself. Messages which are ready to be sent hold the stamped sending envelope icon while messages which are not ready to be sent hold an open envelope icon with the message outside of the envelope (which is the same icon as messages which are saved as a drafts).
There is a clear visual difference between submitted and hanging messages in the Outbox.
Note 1: If your sent message hangs in the Outbox and you cannot remove it, see this guide with tips on how to get rid of them.
Note 2: To learn more about the icons Outlook uses and their meaning, see this list of icons.