[Exim] Exim and DNSBLs

Hi,

I configured my exim 3.36 server to use the following dnsbl's:

rbl_domains = relays.ordb.org/reject : \
              or.orbl.org/reject : \
              sbl.spamhaus.org/reject : \
              opm.blitzed.org/reject

When I start a smtp session exim first checks these DNSBLs. ??
Is this in fact so?
I think it could happen this way because of the long delay
till exim responses with the proposal and start of the smtp-session.
My question is how long can this delay be till the connection is lost?
In special I think of the possibility that other MXs are trying to connect
to our MX and get a timeout because of the DNSBLs checks...
The server I'm talking about is reachable at mail.vit.de (MX=10).

Any ideas?

Thanx,
Michael