DKIM helps prevent spam, and ensure that mail from Gentoo.org is really from Gentoo.org. It's pretty easy to add - just a DNS record for each mail server, and a little mail server config change. Reproducible: Always
No, we have zero requirement for developers to relay their outgoing mail via us. We have the following SPF records in that regard: gentoo.org. 1200 IN TXT "v=spf1 mx ptr ?all" lists.gentoo.org. 60 IN TXT "v=spf1 mx a:lists.gentoo.org a:pigeon.gentoo.org a:robin.gentoo.org include:gentoo.org -all" If the mail claims to be from @lists.gentoo.org, then it really is, because all of that mail comes from one of exactly 3 machines. If it claims to be @gentoo.org, then it could be direct, or it could be from any developer directly (using their own ISPs or own systems).