Forbid logins on the SMTP-STARTTLS port

Also, introduce the outline of a framework to handle message sending
differently to message receipt.

internal/smtp/sender.go

@@ -0,0 +1,21 @@
+package smtp
+
+import (
+	"fmt"
+	"io"
+)
+
+type Sender struct{}
+
+func (s *Sender) Name() string {
+	return "submission-starttls"
+}
+
+// We're looking for the email to be from a logged-in account, to anywhere
+// on the internet - including locally!
+//
+// Should we handle local delivery differently to remote delivery, or connect
+// to ourselves, i.e., from submission, to smtp?
+func (s *Sender) Send(from string, to []string, reader io.Reader) error {
+	return fmt.Errorf("Not yet implemented")
+}