[exim] host lookup did not complete & sender verify

Hi,

I've received more than once error messages
"host lookup did not complete" in processing sender
verifies.

A typical example is the sender-address
c_sshaw@???

There is no MX record and no A record for the
domain (servfail). Exim issues a 451 response which
doesn't really help in this case.

Is there a switch to stop the ping-pong after
a specified time/tries?

Thanks
Reinhard Haller