[MM-36872] add threading headers to notification emails (#18232)

* [MM-36872] Adding headers for mail threading

* MM-36872 Add message id and mail threading headers

* MM-36872 Add message id and mail threading headers

* MM-36872 Add msgId, inreplyto, references headers

* Fixing comment issues

* Applying gofmt lint fixes

* Applying gofmt lint fixes

* MM-36872: Updates required from merge.

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Martin Kraft <martin@upspin.org>

shared/mail/mail.go

@@ -52,6 +52,9 @@ type mailData struct {
 	htmlBody      string
 	embeddedFiles map[string]io.Reader
 	mimeHeaders   map[string]string
+	messageID     string
+	inReplyTo     string
+	references    string
 }
 
 // smtpClient is implemented by an smtp.Client. See https://golang.org/pkg/net/smtp/#Client.
@@ -230,7 +233,7 @@ func TestConnection(config *SMTPConfig) error {
 	return nil
 }
 
-func SendMailWithEmbeddedFilesUsingConfig(to, subject, htmlBody string, embeddedFiles map[string]io.Reader, config *SMTPConfig, enableComplianceFeatures bool, ccMail string) error {
+func SendMailWithEmbeddedFilesUsingConfig(to, subject, htmlBody string, embeddedFiles map[string]io.Reader, config *SMTPConfig, enableComplianceFeatures bool, messageID string, inReplyTo string, references string, ccMail string) error {
 	fromMail := mail.Address{Name: config.FeedbackName, Address: config.FeedbackEmail}
 	replyTo := mail.Address{Name: config.FeedbackName, Address: config.ReplyToAddress}
 
@@ -243,13 +246,16 @@ func SendMailWithEmbeddedFilesUsingConfig(to, subject, htmlBody string, embedded
 		subject:       subject,
 		htmlBody:      htmlBody,
 		embeddedFiles: embeddedFiles,
+		messageID:     messageID,
+		inReplyTo:     inReplyTo,
+		references:    references,
 	}
 
 	return sendMailUsingConfigAdvanced(mail, config)
 }
 
-func SendMailUsingConfig(to, subject, htmlBody string, config *SMTPConfig, enableComplianceFeatures bool, ccMail string) error {
-	return SendMailWithEmbeddedFilesUsingConfig(to, subject, htmlBody, nil, config, enableComplianceFeatures, ccMail)
+func SendMailUsingConfig(to, subject, htmlBody string, config *SMTPConfig, enableComplianceFeatures bool, messageID string, inReplyTo string, references string, ccMail string) error {
+	return SendMailWithEmbeddedFilesUsingConfig(to, subject, htmlBody, nil, config, enableComplianceFeatures, messageID, inReplyTo, references, ccMail)
 }
 
 // allows for sending an email with differing MIME/SMTP recipients
@@ -308,6 +314,18 @@ func SendMail(c smtpClient, mail mailData, date time.Time) error {
 		headers["CC"] = []string{mail.cc}
 	}
 
+	if mail.messageID != "" {
+		headers["Message-ID"] = []string{mail.messageID}
+	}
+
+	if mail.inReplyTo != "" {
+		headers["In-Reply-To"] = []string{mail.inReplyTo}
+	}
+
+	if mail.references != "" {
+		headers["References"] = []string{mail.references}
+	}
+
 	for k, v := range mail.mimeHeaders {
 		headers[k] = []string{encodeRFC2047Word(v)}
 	}