smtp_spec brevity.
Support for static configuration per provider.
Smtp::Postmark has static :port and :authentication.
Smtp::Mandrill provider.
Dynamic config takes precedence over static.
Smtp::Sendgrid provider.
SMTP username => user_name, consistent with Mail, ActionMailer, etc.
Provider#name, and Service#provider is public.
Glimmer of actual usage in README.md