Update SMTP authentication mechanisms and related tests

Introduced the SMTPAuthNoAuth mechanism in the auth.go file meant for instances where authentication is not required or supported. Adjusted tests in client_test.go to account for this new mechanism. Minor order modifications were also made to the list of supported SMTP AUTH types and respective tests for better consistency.

Closes #176

auth.go

@@ -11,15 +11,20 @@ type SMTPAuthType string
 
 // Supported SMTP AUTH types
 const (
+ // SMTPAuthCramMD5 is the "CRAM-MD5" SASL authentication mechanism as described in RFC 4954
+ SMTPAuthCramMD5 SMTPAuthType = "CRAM-MD5"
+
  // SMTPAuthLogin is the "LOGIN" SASL authentication mechanism
  SMTPAuthLogin SMTPAuthType = "LOGIN"
 
+ // SMTPAuthNoAuth is equivalent to performing no authentication at all. It is a convenience
+ // option and should not be used. Instead, for mail servers that do no support/require
+ // authentication, the Client should not be used with the WithSMTPAuth option
+ SMTPAuthNoAuth SMTPAuthType = ""
+
  // SMTPAuthPlain is the "PLAIN" authentication mechanism as described in RFC 4616
  SMTPAuthPlain SMTPAuthType = "PLAIN"
 
- // SMTPAuthCramMD5 is the "CRAM-MD5" SASL authentication mechanism as described in RFC 4954
- SMTPAuthCramMD5 SMTPAuthType = "CRAM-MD5"
-
  // SMTPAuthXOAUTH2 is the "XOAUTH2" SASL authentication mechanism.
  // https://developers.google.com/gmail/imap/xoauth2-protocol
  SMTPAuthXOAUTH2 SMTPAuthType = "XOAUTH2"

client_test.go

@@ -97,6 +97,7 @@ func TestNewClientWithOptions(t *testing.T) {
  {"WithTLSPortPolicy()", WithTLSPortPolicy(TLSOpportunistic), false},
  {"WithTLSConfig()", WithTLSConfig(&tls.Config{}), false},
  {"WithTLSConfig(); config is nil", WithTLSConfig(nil), true},
+ {"WithSMTPAuth(NoAuth)", WithSMTPAuth(SMTPAuthNoAuth), false},
  {"WithSMTPAuth()", WithSMTPAuth(SMTPAuthLogin), false},
  {
  "WithSMTPAuthCustom()",
@@ -576,9 +577,9 @@ func TestSetSMTPAuthCustom(t *testing.T) {
  want string
  sf bool
  }{
- {"SMTPAuth: PLAIN", smtp.PlainAuth("", "", "", ""), "PLAIN", false},
  {"SMTPAuth: CRAM-MD5", smtp.CRAMMD5Auth("", ""), "CRAM-MD5", false},
  {"SMTPAuth: LOGIN", smtp.LoginAuth("", "", ""), "LOGIN", false},
+ {"SMTPAuth: PLAIN", smtp.PlainAuth("", "", "", ""), "PLAIN", false},
  }
  si := smtp.ServerInfo{TLS: true}
  for _, tt := range tests {
@@ -1405,7 +1406,7 @@ func TestXOAuth2OK(t *testing.T) {
  }
  c, err := NewClient("fake.host",
  WithDialContextFunc(getFakeDialFunc(fake)),
- WithTLSPolicy(TLSOpportunistic),
+ WithTLSPortPolicy(TLSOpportunistic),
  WithSMTPAuth(SMTPAuthXOAUTH2),
  WithUsername("user"),
  WithPassword("token"))
@@ -1444,7 +1445,7 @@ func TestXOAuth2Unsupported(t *testing.T) {
  }
  c, err := NewClient("fake.host",
  WithDialContextFunc(getFakeDialFunc(fake)),
- WithTLSPolicy(TLSOpportunistic),
+ WithTLSPortPolicy(TLSOpportunistic),
  WithSMTPAuth(SMTPAuthXOAUTH2))
  if err != nil {
  t.Fatalf("unable to create new client: %v", err)