[Exim] quick Exim and Procmail question

Hi,

When an MTA from somewhere makes the SMTP connection to Exim on my box,
and exim pipes the mail through procmail, does the SMTP TCP connection
stay open until the procmail run finishes?

Or, does Exim receive the message in full, then terminate the SMTP
session, *then* pipe the mail to procmail?

I ask, because my procmail pipes each inbound message through SpamBayes.
I thought that if I set SpamBayes to go into a lengthy sleep (say, 5-10
minutes) when it positively identifies a spam, it might cause some pain
and annoyance to the spammer's MTA on the other end.
--
Cheers
David