Why are there extra domains in the domain sending list?
You might see a domain other than your main store domain in Mandrill under Settings > Sending Domains. For example in the screenshot below you see *gmail.com*.
What this is usually caused by is that you may have had a test email sent from a gmail.com email address. Sometimes the SMTP Pro Magento extension for example sends a test email using firstname.lastname@example.org as the sender email address.
Mandrill detects that as a sending domain and puts it in the list. But if it's not a domain that you actually use for sending, you can simply click *Delete* and not worry about seeing up DKIM / SPF for it.