Add the target.remote.force_ipv4 option

dist/vim/syntax/maddy-conf.vim

@@ -139,6 +139,7 @@ syn keyword maddyModDir
 	\ fail_open
 	\ file
 	\ flags
+	\ force_ipv4
 	\ fs_dir
 	\ fsstore
 	\ full_match

docs/man/maddy-targets.5.scd

@@ -128,6 +128,17 @@ per-source/per-destination are as observed when message exits the server.
 
 Choose the local IP to bind for outbound SMTP connections.
 
+*Syntax*: force_ipv4 _boolean_ ++
+*Default*: false
+
+Force resolving outbound SMTP domains to IPv4 addresses. Some server providers
+do not offer a way to properly set reverse PTR domains for IPv6 addresses; this
+option makes maddy only connect to IPv4 addresses so that its public IPv4 address
+is used to connect to that server, and thus reverse PTR checks are made against
+its IPv4 address.
+
+Warning: this may break sending outgoing mail to IPv6-only SMTP servers.
+
 *Syntax*: connect_timeout _duration_ ++
 *Default*: 5m
 

internal/target/remote/remote.go

@@ -62,6 +62,7 @@ type Target struct {
 	name      string
 	hostname  string
 	localIP   string
+	ipv4      bool
 	tlsConfig *tls.Config
 
 	resolver    dns.Resolver
@@ -107,6 +108,7 @@ func (rt *Target) Init(cfg *config.Map) error {
 
 	cfg.String("hostname", true, true, "", &rt.hostname)
 	cfg.String("local_ip", false, false, "", &rt.localIP)
+	cfg.Bool("force_ipv4", false, false, &rt.ipv4)
 	cfg.Bool("debug", true, false, &rt.Log.Debug)
 	cfg.Custom("tls_client", true, false, func() (interface{}, error) {
 		return &tls.Config{}, nil
@@ -168,6 +170,15 @@ func (rt *Target) Init(cfg *config.Map) error {
 			LocalAddr: addr,
 		}).DialContext
 	}
+	if rt.ipv4 {
+		dial := rt.dialer
+		rt.dialer = func(ctx context.Context, network, addr string) (net.Conn, error) {
+			if network == "tcp" {
+				network = "tcp4"
+			}
+			return dial(ctx, network, addr)
+		}
+	}
 
 	return nil
 }