External SMTP Support+discussion
Wagn's functionality here is at present just a very thin wrapper around Ruby-on-Rails' ActionMailer (which in turn is using an external smtp gem). My guess is that the best answers at present would probably come from the rails community. Perhaps this is helpful:
To translate that into WagnSpeak, you might try something like this in wagn.yml
smtp:
openssl_verify_mode: false
if that doesn't work, then I would suspect that googling the main error message ("hostname does not match the server certificate") would be likely to turn up other options.