How can I determine the email client from email headers?

Question

It seems like most popular email clients don't include the X-Mailer header. Is there any good method to determine if an email was sent by a popular client, such as Gmail, Outlook or Apple Mail?

Holley

Also try the User-Agent header. – Chessman

I don't see the User-Agent header in any of the emails I'm testing – Holley

Answer 1

As mentioned in the comments, the User-Agent header can be used to determine the client MUA.

If this header isn't present in any of the emails being tested, this answer suggests that you could also check other fields like the message-id since some email clients will generate a message-id of their own.

Some more information about different useful headers can be found here.

Cobblestone