Use `net.JoinHostPort` for joining host and port.

go-sendxmpp.go

@@ -8,8 +8,10 @@ import (
 	"bufio"
 	"crypto/tls"
 	"errors"
+	"fmt"
 	"io"
 	"log"
+	"net"
 	"os"
 	"os/exec"
 	"os/user"
@@ -280,7 +282,7 @@ func main() {
 		password = config.password
 		resource = config.resource
 		if config.port != "" {
-			server = server + ":" + config.port
+			server = net.JoinHostPort(server, fmt.Sprint(config.port))
 		}
 	}