[exim] No QUIT is spambot indicator

Not that it is by itself but when combined with other conditions it is
very effective. My theory is that after the message is sent by the virus
sending the quit just takes more time and bandwidth so the spambot just
leaves the connection open on the server side.

But - almost all of the connections that time out are spambots. So you
can combine this with a number of other sins and have a very effective
means of identifying spambots.