Remove superfluous validateLine() calls in sendMail()

Client.SendMail() already calls validateLine() via Client.Mail() and
Client.Rcpt(), so there is no need to have these checks in sendMail().

Having these checks don't hurt as such, but looking at just the
sendMail() function it's not obvious whether these checks are needed if
you want to implement your own SendMail()-type function. With this
change, you can copy/paste sendMail(), add the smtp. package selector,
and modify as needed.

Also removes a superfluous nil check as a bonus.

Author: arp242

client.go

@@ -499,7 +499,7 @@ func (c *Client) Rcpt(to string, opts *RcptOptions) error {
 	sb.Grow(2048)
 	fmt.Fprintf(&sb, "RCPT TO:<%s>", to)
 	if _, ok := c.ext["DSN"]; ok && opts != nil {
-		if opts.Notify != nil && len(opts.Notify) != 0 {
+		if len(opts.Notify) != 0 {
 			sb.WriteString(" NOTIFY=")
 			if err := checkNotifySet(opts.Notify); err != nil {
 				return errors.New("smtp: Malformed NOTIFY parameter value")
@@ -744,15 +744,6 @@ func (c *Client) SendMail(from string, to []string, r io.Reader) error {
 var testHookStartTLS func(*tls.Config) // nil, except for tests
 
 func sendMail(addr string, implicitTLS bool, a sasl.Client, from string, to []string, r io.Reader) error {
-	if err := validateLine(from); err != nil {
-		return err
-	}
-	for _, recp := range to {
-		if err := validateLine(recp); err != nil {
-			return err
-		}
-	}
-
 	var (
 		c   *Client
 		err error