The "exists" mechanism (edit)
Perform an A query on the provided domain. If a result is found, this constitutes a match. It doesn't matter what the lookup result is – it could be 127.0.0.2.
When you use macros with this mechanism, you can perform RBL-style reversed-IP lookups, or set up per-user exceptions.
Examples:
In the following example, the client IP is 1.2.3.4 and the current-domain is example.com.
"v=spf1 exists:example.net -all"
If example.net does not resolve, the result is fail. If it does resolve, this mechanism results in a match.
