Signs a message with DKIM and prints the signed message.
mix mail_auth.sign message.eml --domain example.com --selector s1 --key key.pemOptions:
--domain,--selector,--key(required): the signing domain, the selector, and a file with the private key in PEM--canonicalization:relaxed/relaxed(the default),simple/simple,relaxed/simpleorsimple/relaxed--identity: thei=tag--expires-in: seconds until the signature expires--now: the signing time, in Unix seconds