Where does this "delayed send" capability in Mail come from? My only plugins as far as I know are Mail Act-On and MailTags. Does this functionality comes from them? I could not find anything on their website. Any other idea?
It looks like you must be using the Mail Act-On 3 Mavericks beta.

They have a link on their page to it and some what's new info.


  What is new in Mail Act-On 3
  
  Mavericks Support Mail Act-On 3 now supports OS X 10.8 and 10.9 New
  interface Reply Templates Composer options including setting delivery
  times specifying outbox rules to apply specifying archive mailbox for
  sent mail