When you activate the Sender address verification, the "From:" addresses of incoming emails will be checked for consistency. Without the additional Callout function, it will only be checked if the envelope sender address (also known as "return path") has a valid MX or A record in DNS. With the Callout function activated, the proxy connect to the return-path host and issue a RCPT command to see if the sender address is really deliverable. If that is not the case, the proxy will not accept mail coming from that address.
When you activate the Sender address verification, the "From:" addresses of incoming emails will be checked for consistency. Without the additional Callout function, it will only be checked if the envelope sender address (also known as "return path") has a valid MX or A record in DNS. With the Callout function activated, the proxy connect to the return-path host and issue a RCPT command to see if the sender address is really deliverable. If that is not the case, the proxy will not accept mail coming from that address.
