HELO error repro

hello.go

package main

import (
	"net"
	"net/smtp"
)

func main() {
	conn, err := net.Dial("tcp", "localhost:2500")
	if err != nil {
		panic(err)
	}

	c, err := smtp.NewClient(conn, "localhost:25")
	if err != nil {
		panic(err)
	}

	err = c.Hello("swc") // err is not nil
	if err != nil {
		panic(err)
	}
}