External SMTP

Serves the `smtp` capability socket from either of two credential sources — a granted connection's MINTED smtp credential when one is bound (`-C smtp=<connection>`), or pasted vars otherwise — with no bundled mail server. The pasted path is not a legacy fallback: Mailgun, SendGrid and Postmark have no connection type at all, so for them it is the only path. Point vars at any submission relay (host/port/encryption/ username/password/fromAddress). Presets: AWS SES (email-smtp.<region>.amazonaws.com:587 starttls), Mailgun (smtp.mailgun.org:587 starttls), SendGrid (smtp.sendgrid.net:587 starttls, username "apikey"), Postmark (smtp.postmarkapp.com:587 starttls). Distinct from mailcow: SMTP relay only, no mailbox hosting.