New to TB. Just trying to understand how it all works. In the message pane there are little curved arrow icons (see attached snapshot) before the message header on some, but not all messages. So far, I have only seen them on messages in the local folder I used to receive imported messages from Outlook. Some are blue pointing right and others are purple and pointing left.

I looked for a guide on the message pain and /or the UI, that would help me with this and future discoveries in the new to me program, but found little documentation on the various items on the interface. Most is intuitive, but I can not figure this one out. I am sure it should be obvious, but not to me so far ;) Thanks in advance.