How To use External Message To Send IDOC From XI To R/3

we need to split one source message to several IDOC To

R/3,But Standard Imported r/3 idoc contains only one idoc per message,so we export the standart idoc to a

xsd file and change it to a multi "IDOC" segment structure and import back to xi again,after that,we create a message interface use the imported

message type and use it to send IDOC to r/3 via idoc adapter,but at the "call atapter" step it gernarate a

error:"Unable to interpret IDoc interface MI_IDOC_IN".

what we should pay attention to when use external message

to send idoc to r/3?

(when use the imported idoc message to send single idoc per message,it is no probelm)

thx