News : Mandrill to prevent unauthorized modification of the From header

Now, when you send from a new domain or add a domain manually in your Mandrill account and also set up SPF and DKIM for that domain, you’ll need to verify it. By verifying your domain, you confirm ownership of that domain and prevent other Mandrill accounts from sending mail authenticated by your domain. Once you verify a domain in your account, any other Mandrill user that wants to sign with that domain will also need to go through the verification process. If they don’t verify the domain, the email will still be delivered, but will be signed with a generic mandrillapp.com signature.

https://blog.mandrill.com/we-are-making-domain-verification-mandatory.html

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