[Exim] SMTP Sender-verify callouts

Hi,

I have recently tried using SMTP callout's to verify senders validity. For
the most part it is working fine, however I am having difficulty with some
senders that I know are valid.

When Exim performs a callout does it call back to the sending server's
apparent DNS name or does it connect to the source IP address?

Am I correct in thinking that if I am able to perform a manula SMTP
conversation with the sending SMTP server and send a message by hand that
Exim should be able to verify the recipient in the same way ? Or could I be
misunderstanding?

Thanks,

Anthony Warren
Medway Council