Make "Send" button inactive during MEComposeSessionHandler work.

What if "session:canSendMessageWithCompletionHandler:" method has to take a couple of seconds to make a decision?

User may be confused when he click "Send" button and nothing is happen at once.

Is there a legal way to make "Send" button disabled during MEComposeSessionHandler work?

Or may be there another way to show some progress that my mail extension is working? May be it is possible to show modal window?