>I get a message saying I've reached the limit of 50 entries when trying to mark a message.
>1. How is the number 50 entries calculated? - My marked messages folder is now empty, I saved what was there in a personal folder, and the count is less than 50.
>2. If the number is the total of messages I ever marked (which makes sense), then there must be a list, and a way to un-mark a message, but I couldn't find it...
>Can somebody show me the way?

I have adjusted the message when the limit is reached to provide a better explanation. As Sergey mentioned, the account setup can be used to remove old threads in order to make room for new ones.