how to send scheduled mails with Mail?
Mail.app has no built-in function to send scheduled mails. You can use Automator to achieve the same result, though.

Open Automator and select "New iCal Alarm". Drag "New Mail Message" to the workflow. It will bring up a small composition window, just like in Mail.app. 

Drag "Send Outgoing Messages" to the workflow. You can test it at this point to make sure that it's properly set up.

Save it. iCal should automatically open, and you can drag the event wherever/whenever you like. The message will be sent when the alarm is triggered.