Ah - this is the problem. The DKIM record is syntactically correct but is set up using the wrong key.
The records should look like:
discourse._domainkey.brightstorm.com IN TXT "v=DKIM1\; k=rsa\; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCojtk3fqF69pT6SZcIwoYzjQfdOBTFK7AOyxEGBwHLZ+xqwQQlVgfL6xFZ7FhCYAczkGTCjdChX/qf6dg4LrtXrb+apymj9WpLOwPir6P5Mv9FH3t3BgrQeyyCLhAHqDrUk+kU3B2z1uva3oWw3qN9MLZaX8HjR13w9ywVEgzjpQIDAQAB"
mandrill._domainkey.brightstorm.com IN TXT "v=DKIM1\; k=rsa\; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCrLHiExVd55zd/IQ/J/mRwSRMAocV/hMB3jXwaHH36d9NaVynQFYV8NaWi69c1veUtRzGt7yAioXqLj7Z4TeEUoOLgrKsn8YnckGs9i3B3tVFB+Ch/4mPhXWiNfNdynHWBcPcbJ8kjEQ2U8y78dHZj1YeRXXVvWob2OaKynO8/lQIDAQAB"
The reason the signature check is failing is that our signature is trying to be verified against the mandrill public key - that ain’t gonna fly.
For future reference, the result we get from a DKIM check for a hosted domain should be:
v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCojtk3fqF69pT6SZcI
woYzjQfdOBTFK7AOyxEGBwHLZ+xqwQQlVgfL6xFZ7FhCYAczkGTCjdChX/qf6dg4LrtXrb+apy
mj9WpLOwPir6P5Mv9FH3t3BgrQeyyCLhAHqDrUk+kU3B2z1uva3oWw3qN9MLZaX8HjR13w9ywV
EgzjpQIDAQAB
Version
v= DKIM1
Key type
k= rsa
Public key
p= MIGfMA0GCSqG...jR13w9ywVEgzjpQIDAQAB
If you have trouble, please feel to let us know immediately rather than changing configuration as we can help with the diagnosis. For example, I’m about to PM you the email logs from the first few messages you apparently tried sending and they show they were indeed delivered to gmail. If you’re having deliverability problems beyond that we can help ensure SPF and DKIM are correctly setup.