Sender Policy Framework Spf

DomainTools Releases Mail Server Investigation Site, ReverseMX.com

Says Tim Chen, CEO, DomainTools, “The DomainTools brand has historically been about surfacing and correlating interesting data around domain names and the DNS. The release of ReverseMX.com gets us back to those roots while simultaneously allowing us to experiment with some new database technologies that will help us to serve more data faster. We are looking forward to seeing how our strong community of active users can benefit from our SPF and MX data.”

DomainTools.com Introduces ReverseMX.com, Pinpointing Hard-to-Detect Relationships Between Mail Servers and DomainsUsers Can Analyze MX & SPF Usage and Configuration by Region and or Network, Gaining Fresh Insight into RelationshipsSeattle, WA — August XX, 2011 — DomainTools, the recognized leader in domain name research and monitoring , today announced the launch of ReverseMX.com . The website allows users to analyze MX and SPF usage and configuration by region and or network, helping to uncover relevant relationships.

In order to build the ReverseMX product, DomainTools has researched and gathered intelligence on mail server data related to over 130 million domains. ReverseMX not only uncovers which mail services are the most popular on a domains-in-use basis, it also uniquely correlates relationships between mail servers. This is something no other website is offering today.

ReverseMX can be used by a wide audience — basically anyone interested in researching the footprint of small or large email providers.  For example, users can analyze  which mail servers’ domains are using certain email providers, or how Microsoft’s hosted email is doing against Gmail or Yahoo.

In addition, ReverseMX can be used to discover how mail servers are configured, including looking at their SPF records for any anti-spam or anti-phishing activities. Users can look up their own domain name and see if their records are correct and valid.

SendGrid Documentation » SPF Records: Don't have more than 10 ...

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The Sender Policy Framework (SPF) specification has a limit of 10 on the number of DNS lookups required to fully resolve an SPF record. This limit is in place to prevent SPF lookups from being a useful avenue for Denial of Service attacks. However, it is possible to quickly exceed this limit through the reckless use of the include modifier.

Section 10.1 of the SPF RFC, “Processing Limits”, specifies the following in regards to DNS lookups:

SPF implementations MUST limit the number of mechanisms and modifiers that do DNS lookups to at most 10 per SPF check, including any lookups caused by the use of the “include” mechanism or the “redirect” modifier. If this number is exceeded during a check, a PermError MUST be returned. The “include”, “a”, “mx”, “ptr”, and “exists” mechanisms as well as the “redirect” modifier do count against this limit. The “all”, “ip4″, and “ip6″ mechanisms do not require DNS lookups and therefore do not count against this limit. The “exp” modifier does not count against this limit because the DNS lookup to fetch the explanation string occurs after the SPF record has been evaluated.