problems with two different proposed solutions. However, Sender-ID and SPF yield the same result in approximately 80% of the cases, according to a billion message analysis. Standardization issues The pra has the disadvantage that forwarders and mailing lists can only support it by modifying the mail header, e.g. by inserting a Sender or Resent-Sender. The latter violates RFC 2822 and can be incompatible with RFC 822. With SPF, mailing lists continue to work as is. Forwarders wishing to support SPF only need to modify SMTP MAIL FROM and RCPT TO, not the mail. This concept is not new: with the original RFC 821 SMTP forwarders always added their host name to the reverse path in the MAIL FROM. The most problematic point in the core Sender ID specification is its recommendation to interpret v=spf1 policies like spf2.0/mfrom,pra instead of spf2.0/mfrom. This was never intended by all published SPF drafts since 2003, and for an unknown large number of v=spf1 policies an evaluation for pra could cause bogus results for many cases where pra and mfrom are different. This problem was the basis of an appeal to the Internet Architecture Board (IAB). In response to another prior appeal the IESG already noted that Sender ID cannot advance on the IETF standards track without addressing the incompatibility with a MUST in RFC 2822. Various surveys performed in 2012, when SPF turned from experimental to proposed standard, showed that fewer than 3% of mail domains published specific requests for using the pra, compared to some 40~50% of mail domains using SPF. Patents The Sender ID proposal was the subject of controversy regarding licensing issues: Microsoft holds patents on key parts of Sender ID and used to license those patents under terms that were not compatible with the GNU General Public License and which were considered problematic for free software implementations in general. On October 23, 2006, Microsoft placed those patents under the Open Specification Promise, which is compatible with some free and open source licenses, but not with the most recent version of the GPL license, version 3.x. See also :Category:Email authentication E-mail authentication overview MARID (IETF WG in 2004) DKIM DomainKeys References External links ASF Position Regarding Sender ID statement from the Apache Software Foundation IAB appeal about Sender ID's reuse of v=spf1 for PRA from the SPF project (2006). Debian project unable to deploy Sender ID statement by the Debian project