Split the concrete SMTP struct into separate MSA and MTA implementations

internal/services/services.go

@@ -40,16 +40,16 @@ type concrete struct {
 }
 
 // For now, bind unconditionally to these ports, on IPv4 + IPv6:
-// 25  - incoming SMTP (STARTTLS)
-// 587 - outgoing SMTP (STARTTLS)
+// 25  - SMTP+MTA (STARTTLS)
+// 587 - SMTP+MSA (STARTTLS)
 // 143 - IMAP (STARTTLS)
 // 993 - IMAP (TLS)
 //
 // Each listener gets incoming connections serviced against the passed-in store.
 func (c *concrete) Run() {
 	c.servers = []genServer{
-		smtp.NewServer(c.cancel, c.store, false),
-		smtp.NewServer(c.cancel, c.store, true),
+		smtp.NewMTA(c.cancel, c.store),
+		smtp.NewMSA(c.cancel, c.store),
 
 		imap.NewServer(c.cancel, c.store, false),
 		imap.NewServer(c.cancel, c.store, true),