If your recipients use Outlook, Hotmail (and other Microsoft webmail services), or Gmail, they may see an indication in the email that it was sent by Mandrill on your behalf instead of just your 'from' name and address. Add SPF and DKIM and verify your sending domains to remove the 'on behalf of' or 'via' information.
When you send an email through Mandrill, the email is transmitted through the Mandrill servers instead of your server. Email can be easy to forge, so Mandrill automatically authenticates all emails sent through our system using multiple authentication methods to help improve deliverability.
If your sending domain doesn't have DNS records authorizing Mandrill to send on your behalf, Mandrill adds a Sender header as part of authentication. Some email programs will display this information to recipients so they can see that the email was sent from a different server than the 'from' address.
Once you add the recommended DNS records and verify your domain, the SenderID and DKIM authentication will be handled by your domain instead of Mandrill, and Outlook, Hotmail, and Gmail won't show the on behalf of or via information.
You'll need to add SPF and DKIM records and verify ownership of your sending domains before you can send email through your account. Mandrill will not send any email from unverified domains or domains without valid SPF and DKIM records, including public domains like gmail.com, yahoo.com, and more.
A message that is rejected with the reject reason unsigned indicates that the sending domain hasn't been properly set up, and that your account is unable to send and authenticate email from that domain.