net/smtp: unable to retrieve SMTP server response error information after EHLO command in (*Client).hello() function

What version of Go are you using (go version)?

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

What did you do?

send email using a non-whitelist ip address

What did you expect to see?

function smtp.SendMail return the real error {"Code": 117, "Msg": "The IP Address is not whitelisted: x.x.x.x"} from (*Client).ehlo()

What did you see instead?

function smtp.SendMail return the error EOF from (*Client).helo()

Proposal

Before

The comment of (*Client).helo() is helo sends the HELO greeting to the server. It should be used only when the server does not support ehlo., but in (*Client) hello() the implement is:

// hello runs a hello exchange if needed.
func (c *Client) hello() error {
	if !c.didHello {
		c.didHello = true
		err := c.ehlo()
		if err != nil {
			c.helloError = c.helo()
		}
	}
	return c.helloError
}

When we get a error from (*Client).helo() not only 502 (permitted only with an old-style server that does not support EHLO) (see 4.3.2 Command-Reply Sequences), the c.helo() will still execute and return another error like EOF what I got. When we print error like "EOF" in log, we cannot get any thing helpful to debug.

After

// hello runs a hello exchange if needed.
func (c *Client) hello() error {
   if !c.didHello {
   	c.didHello = true
   	c.helloError = c.ehlo()
   	if c.helloError != nil && c.helloError.(*textproto.Error).Code == 502 {
   		c.helloError = c.helo()
   	}
   }
   return c.helloError
}