http://www.openspf.org/FAQ/Common_mistakes#helo recommends to publish SPF records for HELO names used by mail servers. However: $ host -t TXT smtp.gentoo.org smtp.gentoo.org has no TXT record I'd suggest to add a simple policy like "v=spf1 a -all" there. Presumably, the same applies to pigeon.gentoo.org as well.
Not sure this is actually needed if google isn't doing it. google.com mail is handled by 30 alt2.aspmx.l.google.com. alt2.aspmx.l.google.com has no TXT record alt2.aspmx.l.google.com has no SPF record
Google sends "EHLO gmail.com", and they DO have SPF for that entry.
Implemented