Fix/optimize isTempError method

Not that this particular part of the code is performance critical, but I figured that the `strconv.Atoi()` is actually useless in here.

Since all we want to know is if the error code from the SMTP server is a 4xx error, we can just check the first rune of the returned error. The `Atoi` provides us with no advantage over the simple rune compare (except of taking about 3ns longer to execute)

senderror.go

@@ -7,7 +7,6 @@ package mail
 import (
 	"errors"
 	"fmt"
-	"strconv"
 	"strings"
 )
 
@@ -132,12 +131,5 @@ func (r SendErrReason) String() string {
 // isTempError checks the given SMTP error and returns true if the given error is of temporary nature
 // and should be retried
 func isTempError(e error) bool {
-	ec, err := strconv.Atoi(e.Error()[:3])
-	if err != nil {
-		return false
-	}
-	if ec >= 400 && ec <= 500 {
-		return true
-	}
-	return false
+	return e.Error()[0] == '4'
 }