Add POST unsubscribe link to the email headers

README.md

@@ -43,11 +43,12 @@ To send email user need to create a sender first and then use `Send` method. The
 - parameters (`email.Params`)
     ```go
     type Params struct {
-        From         string        // From email field
-        To           []string      // From email field
-        Subject      string        // Email subject
-        Attachments  []string      // Attachments
-        InlineImages []string      // Embedding directly to email body. Autogenerated Content-Id (cid) equals to file name
+        From            string   // From email field
+        To              []string // From email field
+        Subject         string   // Email subject
+        UnsubscribeLink string   // POST, https://support.google.com/mail/answer/81126 -> "Use one-click unsubscribe"
+        Attachments     []string // Attachments path
+        InlineImages    []string // Embedding directly to email body. Autogenerated Content-Id (cid) equals to file name
     }
     ```
 

email.go

@@ -42,11 +42,12 @@ type Sender struct {
 
 // Params contains all user-defined parameters to send emails
 type Params struct {
-	From         string   // From email field
-	To           []string // From email field
-	Subject      string   // Email subject
-	Attachments  []string // Attachments path
-	InlineImages []string // InlineImages images path
+	From            string   // From email field
+	To              []string // From email field
+	Subject         string   // Email subject
+	UnsubscribeLink string   // POST, https://support.google.com/mail/answer/81126 -> "Use one-click unsubscribe"
+	Attachments     []string // Attachments path
+	InlineImages    []string // InlineImages images path
 }
 
 // Logger is used to log errors and debug messages
@@ -212,6 +213,11 @@ func (em *Sender) buildMessage(text string, params Params) (message string, err
 	message = addHeader(message, "To", strings.Join(params.To, ","))
 	message = addHeader(message, "Subject", mime.BEncoding.Encode("utf-8", params.Subject))
 
+	if params.UnsubscribeLink != "" {
+		message = addHeader(message, "List-Unsubscribe-Post", "List-Unsubscribe=One-Click")
+		message = addHeader(message, "List-Unsubscribe", "<"+params.UnsubscribeLink+">")
+	}
+
 	withAttachments := len(params.Attachments) > 0
 	withInlineImg := len(params.InlineImages) > 0
 

email_test.go

@@ -299,13 +299,14 @@ func TestEmail_buildMessageWithMIME(t *testing.T) {
 	e := NewSender("localhost", ContentType("text/html"))
 
 	msg, err := e.buildMessage("this is a test\n12345\n", Params{
-		From:    "from@example.com",
-		To:      []string{"to@example.com"},
-		Subject: "non-ascii symbols: Привет",
+		From:            "from@example.com",
+		To:              []string{"to@example.com"},
+		Subject:         "non-ascii symbols: Привет",
+		UnsubscribeLink: "https://example.com/unsubscribe",
 	})
 	require.NoError(t, err)
 	assert.Contains(t, msg, "Content-Transfer-Encoding: quoted-printable\nContent-Type: text/html; charset=\"UTF-8\"", msg)
-	assert.Contains(t, msg, "From: from@example.com\nTo: to@example.com\nSubject: =?utf-8?b?bm9uLWFzY2lpIHN5bWJvbHM6INCf0YDQuNCy0LXRgg==?=\nMIME-version: 1.0", msg)
+	assert.Contains(t, msg, "From: from@example.com\nTo: to@example.com\nSubject: =?utf-8?b?bm9uLWFzY2lpIHN5bWJvbHM6INCf0YDQuNCy0LXRgg==?=\nList-Unsubscribe-Post: List-Unsubscribe=One-Click\nList-Unsubscribe: <https://example.com/unsubscribe>\nMIME-version: 1.0", msg)
 	assert.Contains(t, msg, "\n\nthis is a test\r\n12345\r\n", msg)
 	assert.Contains(t, msg, "Date: ", msg)
 }