A mail-relay problem

[Namik Gunel <namik@knidos.cc.metu.edu.tr>]

      Hi,
      We are the service porvider of Internet in Turkey.
 There are many sites connected to us and many of them are connected 
 by TCP/IP over X.25.And we do not call them to connect, instead they
 call us when they wish to establish connection.
      If any site tries to make smtp connection to such hosts when they 
are connected to us, there is no problem. If they are not connected, mails 
are queued and even returnes back to sender at the site that tries to make 
smtp connection.
      I want to let any of our workstations receive such mails (those 
 are not in its domain) and wait until such hosts connect to us to forward
 them.I can have that workstation receive those mails using mx records but
 how can I make those mails wait until the host (that the mail is supposed
 to arrive at) establishes connection?

      I wish to do this using "zmail" if possible.
      Could you please help me to solve the problem ?
      Any comments are welcome.
 Thanks.
 
 Namik Kemal Gunel
 Middle East Technical University
 Computer Center - Network Group
 namik@knidos.cc.metu.edu.tr