Add In-reply-to header for message grouping

README.md

@@ -47,6 +47,7 @@ To send email user need to create a sender first and then use `Send` method. The
         To              []string // From email field
         Subject         string   // Email subject
         UnsubscribeLink string   // POST, https://support.google.com/mail/answer/81126 -> "Use one-click unsubscribe"
+        InReplyTo       string   // Identifier for email group (category), used for email grouping
         Attachments     []string // Attachments path
         InlineImages    []string // Embedding directly to email body. Autogenerated Content-Id (cid) equals to file name
     }

email.go

@@ -46,6 +46,7 @@ type Params struct {
 	To              []string // From email field
 	Subject         string   // Email subject
 	UnsubscribeLink string   // POST, https://support.google.com/mail/answer/81126 -> "Use one-click unsubscribe"
+	InReplyTo       string   // Identifier for email group (category), used for email grouping
 	Attachments     []string // Attachments path
 	InlineImages    []string // InlineImages images path
 }
@@ -218,6 +219,10 @@ func (em *Sender) buildMessage(text string, params Params) (message string, err
 		message = addHeader(message, "List-Unsubscribe", "<"+params.UnsubscribeLink+">")
 	}
 
+	if params.InReplyTo != "" {
+		message = addHeader(message, "In-reply-to", "<"+params.InReplyTo+">")
+	}
+
 	withAttachments := len(params.Attachments) > 0
 	withInlineImg := len(params.InlineImages) > 0
 

email_test.go

@@ -303,10 +303,11 @@ func TestEmail_buildMessageWithMIME(t *testing.T) {
 		To:              []string{"[email protected]"},
 		Subject:         "non-ascii symbols: Привет",
 		UnsubscribeLink: "https://example.com/unsubscribe",
+		InReplyTo:       "[email protected]",
 	})
 	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: [email protected]\nTo: [email protected]\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, "From: [email protected]\nTo: [email protected]\nSubject: =?utf-8?b?bm9uLWFzY2lpIHN5bWJvbHM6INCf0YDQuNCy0LXRgg==?=\nList-Unsubscribe-Post: List-Unsubscribe=One-Click\nList-Unsubscribe: <https://example.com/unsubscribe>\nIn-reply-to: <[email protected]>\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)
 }