What is the difference between flag message , and Block Sender ? Just Curious .

Flag message puts a red flag on your message so that you can see you need to do something with it later on.

Block sender will stop the person who sent you the message from sending anything else and will place them on your 'blocked sender' list.

Thanks .. Do Something like what ?

You might want to make a call, to file the message or maybe even to send a reply - flag messages is particularly useful if you are checking your email quickly and don't have time to follow up any actions you need to take, or if you are checking through all of your email at once, and want to go back to it and file/reply/call/take some action on the item of email. It's just like sticking a 'post it' note on a paper memo to remind you to do something with it later.