Added the LOGIN authentication mechanism support

Closes #16.

README.md

@@ -76,6 +76,16 @@ bypass the verification of the server's certificate chain and host name by using
 
 Note, however, that this is insecure and should not be used in production.
 
+### 504 5.7.4 Unrecognized authentication type
+
+If you get this error, you should try using the LOGIN authentication mechanism:
+
+    addr := "smtp.example.com:587"
+    auth := gomail.LoginAuth("username", "password", "smtp.example.com")
+    mailer := gomail.NewCustomMailer(addr, auth)
+
+See [issue #16](https://github.com/go-gomail/gomail/issues/16).
+
 
 ## Contact
 

login.go

@@ -0,0 +1,54 @@
+package gomail
+
+import (
+	"errors"
+	"fmt"
+	"net/smtp"
+	"strings"
+)
+
+type loginAuth struct {
+	username string
+	password string
+	host     string
+}
+
+// LoginAuth returns an Auth that implements the LOGIN authentication mechanism.
+func LoginAuth(username, password, host string) smtp.Auth {
+	return &loginAuth{username, password, host}
+}
+
+func (a *loginAuth) Start(server *smtp.ServerInfo) (string, []byte, error) {
+	if !server.TLS {
+		advertised := false
+		for _, mechanism := range server.Auth {
+			if mechanism == "LOGIN" {
+				advertised = true
+				break
+			}
+		}
+		if !advertised {
+			return "", nil, errors.New("gomail: unencrypted connection")
+		}
+	}
+	if server.Name != a.host {
+		return "", nil, errors.New("gomail: wrong host name")
+	}
+	return "LOGIN", nil, nil
+}
+
+func (a *loginAuth) Next(fromServer []byte, more bool) ([]byte, error) {
+	if !more {
+		return nil, nil
+	}
+
+	command := strings.ToLower(strings.TrimSuffix(string(fromServer), ":"))
+	switch command {
+	case "username":
+		return []byte(fmt.Sprintf("%s", a.username)), nil
+	case "password":
+		return []byte(fmt.Sprintf("%s", a.password)), nil
+	default:
+		return nil, fmt.Errorf("gomail: unexpected server challenge: %s", command)
+	}
+}

login_test.go

@@ -0,0 +1,66 @@
+package gomail
+
+import (
+	"net/smtp"
+	"testing"
+)
+
+type output struct {
+	proto string
+	data  []string
+	err   error
+}
+
+const (
+	testUser = "user"
+	testPwd  = "pwd"
+)
+
+func TestPlainAuth(t *testing.T) {
+	tests := []struct {
+		serverProtos     []string
+		serverChallenges []string
+		proto            string
+		data             []string
+	}{
+		{
+			serverProtos:     []string{"LOGIN"},
+			serverChallenges: []string{"Username:", "Password:"},
+			proto:            "LOGIN",
+			data:             []string{"", testUser, testPwd},
+		},
+	}
+
+	for _, test := range tests {
+		auth := LoginAuth(testUser, testPwd, testHost)
+		server := &smtp.ServerInfo{
+			Name: testHost,
+			TLS:  true,
+			Auth: test.serverProtos,
+		}
+		proto, toServer, err := auth.Start(server)
+		if err != nil {
+			t.Fatalf("Start error: %v", err)
+		}
+		if proto != test.proto {
+			t.Errorf("Invalid protocol, got %q, want %q", proto, test.proto)
+		}
+
+		i := 0
+		got := string(toServer)
+		if got != test.data[i] {
+			t.Errorf("Invalid response, got %q, want %q", got, test.data[i])
+		}
+		for _, challenge := range test.serverChallenges {
+			toServer, err = auth.Next([]byte(challenge), true)
+			if err != nil {
+				t.Fatalf("Auth error: %v", err)
+			}
+			i++
+			got = string(toServer)
+			if got != test.data[i] {
+				t.Errorf("Invalid response, got %q, want %q", got, test.data[i])
+			}
+		}
+	}
+}