smtp.Dial（“ ASPMX.L.GOOGLE.COM:25”）连接错误； 但是油灰连接有效

When using Go and smtp.Dial, or even net.Dial, I get the error:

dial tcp 64.233.169.27:25: ConnectEx tcp: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond.

From this code:

mxClient, err := smtp.Dial("ASPMX.L.GOOGLE.COM:25")
if err != nil {
    fmt.Println(err)
}

What I don't understand is that I can connect and send commands (HELO, etc) using putty on port 25 without TLS. If it's a limitation of the package not able to make the connection, is there a recommended way to make a raw socket connection like putty in Go?

I don't see any error on my machine.

package main

import (
        "fmt"
        "net/smtp"
)

func main() {
        mxClient, err := smtp.Dial("ASPMX.L.GOOGLE.COM:25")
        if err != nil {
                fmt.Println(err)
        }
        fmt.Printf("%#v", mxClient)
}

gives

&smtp.Client{Text:(*textproto.Conn)(0xc208074000), conn:(*net.TCPConn)(0xc20802c018), tls:false, serverName:"ASPMX.L.GOOGLE.COM", ext:map[string]string(nil), auth:[]string(nil), localName:"localhost", didHello:false, helloError:error(nil)}

Your ISP might be blocking port 25. I have to use port 587 to googles smtp server to connect.