[Exim] second newbie-question

Hello,

so far I have found out how to check a hosts IP-address using
reverse lookups.
verify = reverse_host_lookup

I still seem to be unable to configure exim 4.34 to use DNS-
lookups for the domain in the "mail from"-command.

any further hints on the correct wording for the verify
statement?

thanks and regards
Christoph