Allow mailer to use SendGrid and not force it to use bring your own SMTP when using that

We use BYO SMTP in order to make our ticket/invoice and other communications more credible.

Mailer/mailings are different, and we’d prefer to use Sendgrid/default mailing for this. Please allow.

This would tie in with another feature as well, which is multiple outboxes (invoice, mailer, etc. could be options with selectable out methods (BYO SMTP, Sendgrid, etc.).