Implementations and Variations
OpenDKIM and MDaemon by Alt-N Technologies have been among the first software implementations of VBR. OpenDKIM provides a milter as well as a standalone library. Roaring Penguin Software's CanIt anti-spam filter supports VBR as of version 7.0.8 released on 2010-11-09.
Spamhaus has released The Spamhaus Whitelist that includes a domain based whitelist, the DWL, where a domain name can be queried as, e.g., dwltest.com._vouch.dwl.spamhaus.org. Although the standard only specifies TXT resource records, following a long established DNSBL practice, Spamhaus has also assigned A resource records with values 127.0.2.0/24 for whitelist return codes. The possibility to query an address may allow easier deployment of existing code. However, their techfaq recommends checking the domain (the value of the d= tag) of a valid DKIM-Signature by querying the corresponding TXT record, and their howto gives details about inserting VBR-Info header fields in messages signed by whitelisted domains.
Read more about this topic: Vouch By Reference
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