[exim] Client SMTP keepalive methods?

    Is there a way for exim to do an SMTP level "keepalive" to clients
during long data scanning (ie.spam, virus, etc)?  

    For example, an Outlook client sending a message that takes longer
than its default 1 minute to process will cause Outlook to think the message
was not sent while exim really has accepted it.  The result is that the
client keeps sending the message over and over again, timing out each time,
and we keep delivering it.

    In this thread:

    http://www.mhonarc.org/archive/html/ietf-smtp/2005-09/msg00119.html

    A technique is described where the server would send back 150
messages to the client.  

    Does Exim have something equivalent?  

    Or is there a recommended practice for dealing with this issue that
doesn't involve accepting the message and scanning later?