[exim-dev] RCPT TO verification

Hi there,

We are a large ISP and we have noticed some Exim implementations
issue RCPT TO back to us on our RCPT TO to verify the sender.

They do this by establishing a separate connection to our MX -- presumably
as an alternative to SPF -- in order to verify the authenticity of the
sender.

If this is an Exim feature, where can I get documentation on how this
works?

I expect this could create a recursive loop if two mail servers that both
implement this feature send mail between one another. How is a loop avoided?

Are there any blogs, caveates, discussions about this kind of behaviour in
general?

With kind regards

-paul