Check if your Gmail message is authenticated

Important:

• Do not use the DKIM length tag (l=) in message headers. This tag makes messages vulnerable to spoofing.
• If a message you sent arrived with a question mark "?" next to your email address, the message wasn't authenticated.

Messages must be authenticated to make sure they're classified correctly. Also, unauthenticated messages are very likely to get rejected. Because spammers can also authenticate mail, authentication by itself isn't enough to guarantee your messages can be delivered.

Make sure messages you sent are authenticated using DKIM (preferred) or SPF.

You can use these steps to prevent your emails from being blocked by Gmail:

• Use RSA keys that are at least 1024-bits long. Emails signed with less than 1024-bit keys are considered unsigned and can easily be spoofed.
• Gmail combines user reports and other signals, with authentication information, when classifying messages. Authentication is mandatory for every mail sender to ensure that your messages are correctly classified. 
• Learn how to create a policy to help control unauthenticated mail from your domain.