Can't you give Mandrill your DKIM private key? Of course that's probably not wise. In which case I'd setup a sister domainname for this purpose.

DKIMs seem interesting. Though without Secure DNS I'm not sure what good it really does, the Public Key listed in non Secure DNS can be spoofed by a MITM attack - though I'm not sure how much of a real concern that is.

True. But a lot of email systems adopted the SPF record type and would not accept email from you if your system wasn't set up with one. I ran into that a few times before actually setting it up.

I always wondered why they just didn't use a TXT record for it, lol...