[Freeswitch-users] Chatplan: is msg received success?

Hi, I use Freeswitch to handle SIP message between SIP extentions, how can server know does the message be received by client or not? I noticed even if a SIP extention is offline/unregister, SIP message will still be sent by server but just the receiver will never get it (even if later the receiver online/registered again). What I want to achieve is: send SIP message to an extention, if received OK, nothing happen; if not, forward the message to user's email address. brgds, Alex