Email authentication
- If a message was correctly DKIM signed, a 'signed-by' header with the sending domain will appear.
- If a message was SPF authenticated, a 'mailed-by' header with the domain name will appear.
- If no authentication information exists, there will be no signed-by or mailed-by headers.
If you're a sending domain
Messages with DKIM signatures use a key to sign messages. Messages signed with short keys can be easily spoofed (see http://www.kb.cert.org/vuls/id/268267), so a message signed with a short key is no longer an indication that the message is properly authenticated. To best protect our users, Gmail will begin treating emails signed with less than 1024-bit keys as unsigned, starting in January 2013. We highly recommend that all senders using short keys switch to RSA keys that are at least 1024-bits long.[1]