From e88e952fccccf4815b629ec1af0b024bbba74a78 Mon Sep 17 00:00:00 2001
From: Robert Sesek <rsesek@bluestatic.org>
Date: Sat, 11 May 2024 16:55:31 -0400
Subject: [PATCH] Begin refactoring out relay logic

---
 smtp/relay_smtp.go | 84 ++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 73 insertions(+), 11 deletions(-)

diff --git a/smtp/relay_smtp.go b/smtp/relay_smtp.go
index 9bee431..15898ac 100644
--- a/smtp/relay_smtp.go
+++ b/smtp/relay_smtp.go
@@ -8,6 +8,7 @@ package smtp
 
 import (
 	"crypto/tls"
+	"fmt"
 	"net"
 	"net/smtp"
 
@@ -44,37 +45,55 @@ func (m *smtpMTA) RelayMessage(env Envelope) {
 }
 
 func (m *smtpMTA) relayMessageToHost(env Envelope, log *zap.Logger, to, host, port string) {
-	from := env.MailFrom.Address
-	hostPort := net.JoinHostPort(host, port)
-	log = log.With(zap.String("host", hostPort))
+	log = log.With(zap.String("host", net.JoinHostPort(host, port)))
 
-	c, err := smtp.Dial(hostPort)
+	c, err := m.dialSMTP(host, port, false)
 	if err != nil {
 		// TODO - retry, or look at other MX records
-		m.RelayFailure(env, log, to, "failed to dial host", err)
+		m.RelayFailure(env, log, to, "failed to connect to MTA", err)
 		return
 	}
 	defer c.Quit()
 
+	m.sendEnvelope(c, to, env, log)
+}
+
+// dialSMTP establishes a connection to the SMTP server at `host`:`port`, attempting
+// to establish a TLS connection. If `requireTLS` is true and the connection cannot
+// be encrypted, returns an error.
+func (m *smtpMTA) dialSMTP(host string, port string, requireTLS bool) (*smtp.Client, error) {
+	c, err := smtp.Dial(net.JoinHostPort(host, port))
+	if err != nil {
+		return nil, err
+	}
+
 	if err = c.Hello(m.server.Name()); err != nil {
-		m.RelayFailure(env, log, to, "failed to HELO", err)
-		return
+		return nil, err
 	}
 
 	if hasTls, _ := c.Extension("STARTTLS"); hasTls {
 		config := &tls.Config{ServerName: host}
 		if err = c.StartTLS(config); err != nil {
-			m.RelayFailure(env, log, to, "failed to STARTTLS", err)
-			return
+			return nil, fmt.Errorf("failed to STARTTLS: %v", err)
 		}
+	} else if requireTLS {
+		return nil, fmt.Errorf("server does not support TLS")
 	}
 
-	if err = c.Mail(from); err != nil {
+	return c, nil
+}
+
+// sendEnvelope writes an Envelope for a specific `to` recipient on an active
+// SMTP client.
+func (m *smtpMTA) sendEnvelope(c *smtp.Client, to string, env Envelope, log *zap.Logger) {
+	from := env.MailFrom.Address
+
+	if err := c.Mail(from); err != nil {
 		m.RelayFailure(env, log, to, "failed MAIL FROM", err)
 		return
 	}
 
-	if err = c.Rcpt(to); err != nil {
+	if err := c.Rcpt(to); err != nil {
 		m.RelayFailure(env, log, to, "failed to RCPT TO", err)
 		return
 	}
@@ -97,3 +116,46 @@ func (m *smtpMTA) relayMessageToHost(env Envelope, log *zap.Logger, to, host, po
 		return
 	}
 }
+
+type smtpRelayMTA struct {
+	smtpMTA
+	host, port string
+	user       string
+	password   string
+}
+
+func NewSMTPRelayMTA(server Server, log *zap.Logger, host, port, user, password string) MTA {
+	return &smtpRelayMTA{
+		smtpMTA: smtpMTA{
+			mta: mta{
+				server: server,
+				log:    log,
+			},
+		},
+		host:     host,
+		port:     port,
+		user:     user,
+		password: password,
+	}
+}
+
+func (m *smtpRelayMTA) RelayMessage(env Envelope) {
+	log := m.log.With(zap.String("host", net.JoinHostPort(m.host, m.port)))
+
+	c, err := m.dialSMTP(m.host, m.port, true)
+	if err != nil {
+		m.RelayFailure(env, log, "", "failed to connect to MTA", err)
+		return
+	}
+	defer c.Quit()
+
+	auth := smtp.PlainAuth("", m.user, m.password, m.host)
+	if err = c.Auth(auth); err != nil {
+		m.RelayFailure(env, log, "", "failed to authenticate to relay", err)
+		return
+	}
+
+	for _, rcptTo := range env.RcptTo {
+		m.sendEnvelope(c, rcptTo.Address, env, log)
+	}
+}
-- 
2.52.0