target/smtp: Do not attempt STARTTLS by default for LMTP connections

Security for LMTP should be configured using Implicit TLS that is
already supported using tls:// scheme for addresses. TLS
is also unnecessary for most LMTP use-cases (Unix sockets, etc) and
attempting STARTTLS caused issues in the past (see foxcpp#309).

docs/man/maddy-targets.5.scd

@@ -351,10 +351,10 @@ Enable verbose logging.
 Advanced TLS client configuration options. See *maddy-tls*(5) for details.
 
 *Syntax*: attempt_starttls _boolean_ ++
-*Default*: yes
+*Default*: yes (no for target.lmtp)
 
 Attempt to use STARTTLS if it is supported by the remote server.
-If TLS handshake fails, connection will be retried without STARTTLS use
+If TLS handshake fails, connection will be retried without STARTTLS
 unless 'require_tls' is also specified.
 
 *Syntax*: require_tls _boolean_ ++

docs/third-party/dovecot.md

@@ -28,7 +28,6 @@ Add `local_mailboxes` block to maddy config using `target.lmtp` module:
 ```
 target.lmtp local_mailboxes {
     targets unix:///var/run/maddy/dovecot-lmtp.sock
-    attempt_starttls no
 }
 ```
 

internal/target/smtp/smtp_downstream.go

@@ -87,7 +87,7 @@ func (u *Downstream) Init(cfg *config.Map) error {
 	var targetsArg []string
 	cfg.Bool("debug", true, false, &u.log.Debug)
 	cfg.Bool("require_tls", false, false, &u.requireTLS)
-	cfg.Bool("attempt_starttls", false, true, &u.attemptStartTLS)
+	cfg.Bool("attempt_starttls", false, !u.lmtp, &u.attemptStartTLS)
 	cfg.String("hostname", true, true, "", &u.hostname)
 	cfg.StringList("targets", false, false, nil, &targetsArg)
 	cfg.Custom("auth", false, false, func() (interface{}, error) {