It would make it easier if we could check mail that has already been read as unread.  

Sometimes I pop on to read it and respond later if it is a personal pm. Just be easier to keep track of what I need to go back and respond to.

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While your idea is good, and it is a common feature on all email programs, and I would like to see TER implement it, there is a solution available now. In TER's PM system you can create folders. Just create a new folder, and move the PMs that you want to respond to later, into it. Once the folder is created it's a simple click. You receive a PM and read it and decide you want to reply but not right now, just click on "move to folder." Later when you have time, you go to that folder and start replying to the messages. You don't have to scroll down your inbox searching for the messages you wanted to reply to, and they don't get lost.

There is also another solution. Just click on reply, then click "save as draft." Later when you have time, you go to your draft folder and actually write the reply