[Exim] Sender verify callback timeouts?

I'm seeing quite a few "temporarily unable to verify sender address" in my
logs from two different mail servers I manage, and it appears to be caused
by using Sender Verify callbacks with MTAs that respond very slowly.

The most recent example was for domain semaphorecorp.com, with a single MX
listed as nic.scruz.net. When I open a port-25 connection to this server, it
takes between 15 and 20 seconds before the SMTP banner is presented. I
assume that Exim is timing out this connection, and thus giving a temporary
sender verify failure.

This is all using Exim 3.22, BTW. Is there any way to manipulate Exim's
timeout for this type of connection?