for localserve, validate the incoming message

it must be completely parsable.
normally, if we receive a message that we cannot fully parse, we accept it and
treat it as opaque application/octet-stream.

also make it more clear that localserve accepts email intended for any email
address.

doc.go

@@ -307,8 +307,8 @@ automatically initialized with configuration files, an account with email
 address mox@localhost and password moxmoxmox, and a newly generated self-signed
 TLS certificate.
 
-All incoming email is accepted (if checks pass), unless the recipient localpart
-ends with:
+All incoming email to any address is accepted (if checks pass), unless the
+recipient localpart ends with:
 
 - "temperror": fail with a temporary error code
 - "permerror": fail with a permanent error code

localserve.go

@@ -47,8 +47,8 @@ automatically initialized with configuration files, an account with email
 address mox@localhost and password moxmoxmox, and a newly generated self-signed
 TLS certificate.
 
-All incoming email is accepted (if checks pass), unless the recipient localpart
-ends with:
+All incoming email to any address is accepted (if checks pass), unless the
+recipient localpart ends with:
 
 - "temperror": fail with a temporary error code
 - "permerror": fail with a permanent error code
@@ -120,7 +120,7 @@ during those commands instead of during "data".
 	golog.Printf("the default admin password is moxadmin")
 	golog.Printf("port numbers are those common for the services + 1000")
 	golog.Printf("tls uses generated self-signed certificate %s", filepath.Join(dir, "localhost.crt"))
-	golog.Printf("all incoming email is accepted (if checks pass), unless the recipient localpart ends with:")
+	golog.Printf("all incoming email to any address is accepted (if checks pass), unless the recipient localpart ends with:")
 	golog.Print("")
 	golog.Printf(`- "temperror": fail with a temporary error code.`)
 	golog.Printf(`- "permerror": fail with a permanent error code.`)

smtpserver/server.go

@@ -1515,6 +1515,18 @@ func (c *conn) cmdData(p *parser) {
 		}
 	}
 
+	if Localserve {
+		// Require that message can be parsed fully.
+		p, err := message.Parse(dataFile)
+		if err == nil {
+			err = p.Walk(nil)
+		}
+		if err != nil {
+			// ../rfc/6409:541
+			xsmtpUserErrorf(smtp.C554TransactionFailed, smtp.SeMsg6Other0, "malformed message: %v", err)
+		}
+	}
+
 	// Prepare "Received" header.
 	// ../rfc/5321:2051 ../rfc/5321:3302
 	// ../rfc/5321:3311 ../rfc/6531:578