Why I get this status SendMsg:WAITMDN ?

Hello, All. I come across a trouble. When I send a EDI file via AS2, and I
get the MDN from my partner, what’s more, the Original-Message-ID in
MDN matches the docID of the EDIINT file I send, but the User Status is
‘SendMsg:WAITMDN’.Why the user status is not ‘Done’ ?And could you tell me in which service the user status is changed ?
thank you

The wm.EDIINT.rules:processMDN service will gets invoked and finally it changes the status as “System status is DONE; user status is ProcessMDNMsgDONE”