I thought about one thing recenent two days.
If a user A in the BBS site B want to sent a instant message to the user D in the BBS site C.
How can I do this?

Current BBS Systems don't sent a instant message across BBS sites.
Even through the MSN, Yahoo or ICQ don't also sent a instant message across them.

So, I thought a method.

       BBS B  -------------- BBS C     
       /                                     \
 User A                              User D   


If user A want to sent a instant message to user D,the method can follow the steps：

1. User A is typing words "Hi,this is Go"
2. BBS B receives the words and  sents a request to test user D in the BBS C is exist or is't exist.
3. Then,BBS C responds this request
4. BBS B receives the response,if answser is no,BBS B tells user A, user D is't exist and BBS B delete the istant message.
   
5. If answers is yes,BBS B delivers the instant message to BBS C
6. BBS C delivers the instant message to user D