Add: Ability to specify delivery status notification (#87)

* Add: Ability to specify delivery status notification when supported by
the server.

* refactor support for DSN

---------

Co-authored-by: Santiago De la Cruz <[email protected]>

README.md

@@ -78,6 +78,7 @@ Go Simple Mail supports:
 - Support add a List-Unsubscribe header (since v2.11.0)
 - Support to add a DKIM signarure (since v2.11.0)
 - Support to send using custom connection, ideal for proxy (since v2.15.0)
+- Support Delivery Status Notification (DSN) (since v2.16.0)
 
 ## Documentation
 
@@ -203,6 +204,9 @@ func main() {
 	// add inline
 	email.Attach(&mail.File{FilePath: "/path/to/image.png", Name:"Gopher.png", Inline: true})
 
+	// also you can set Delivery Status Notification (DSN) (only is set when server supports DSN)
+	email.SetDSN([]mail.DSN{mail.SUCCESS, mail.FAILURE}, false)
+
 	// you can add dkim signature to the email. 
 	// to add dkim, you need a private key already created one.
 	if privateKey != "" {

email.go

@@ -18,22 +18,24 @@ import (
 
 // Email represents an email message.
 type Email struct {
-	from                  string
-	sender                string
-	replyTo               string
-	returnPath            string
-	recipients            []string
-	headers               textproto.MIMEHeader
-	parts                 []part
-	attachments           []*File
-	inlines               []*File
-	Charset               string
-	Encoding              encoding
-	Error                 error
-	SMTPServer            *smtpClient
-	DkimMsg               string
-	AllowDuplicateAddress bool
-	AddBccToHeader        bool
+	from                      string
+	sender                    string
+	replyTo                   string
+	returnPath                string
+	recipients                []string
+	headers                   textproto.MIMEHeader
+	parts                     []part
+	attachments               []*File
+	inlines                   []*File
+	Charset                   string
+	Encoding                  encoding
+	Error                     error
+	SMTPServer                *smtpClient
+	DkimMsg                   string
+	AllowDuplicateAddress     bool
+	AddBccToHeader            bool
+	preserveOriginalRecipient bool
+	dsn                       []DSN
 }
 
 /*
@@ -59,10 +61,13 @@ type SMTPServer struct {
 
 // SMTPClient represents a SMTP Client for send email
 type SMTPClient struct {
-	mu          sync.Mutex
-	Client      *smtpClient
-	KeepAlive   bool
-	SendTimeout time.Duration
+	mu                        sync.Mutex
+	Client                    *smtpClient
+	SendTimeout               time.Duration
+	KeepAlive                 bool
+	hasDSNExt                 bool
+	preserveOriginalRecipient bool
+	dsn                       []DSN
 }
 
 // part represents the different content parts of an email body.
@@ -158,6 +163,34 @@ func (at AuthType) String() string {
 	}
 }
 
+/*
+	DSN notifications
+
+- 'NEVER' under no circumstances a DSN must be returned to the sender. If you use NEVER all other notifications will be ignored.
+
+- 'SUCCESS' will notify you when your mail has arrived at its destination.
+
+- 'FAILURE' will arrive if an error occurred during delivery.
+
+- 'DELAY' will notify you if there is an unusual delay in delivery, but the actual delivery's outcome (success or failure) is not yet decided.
+
+see https://tools.ietf.org/html/rfc3461 See section 4.1 for more information about NOTIFY
+*/
+type DSN int
+
+const (
+	NEVER DSN = iota
+	FAILURE
+	DELAY
+	SUCCESS
+)
+
+var dsnTypes = [...]string{"NEVER", "FAILURE", "DELAY", "SUCCESS"}
+
+func (dsn DSN) String() string {
+	return dsnTypes[dsn]
+}
+
 // NewMSG creates a new email. It uses UTF-8 by default. All charsets: http://webcheatsheet.com/HTML/character_sets_list.php
 func NewMSG() *Email {
 	email := &Email{
@@ -613,6 +646,20 @@ func (email *Email) AddAlternativeData(contentType ContentType, body []byte) *Em
 	return email
 }
 
+// SetDSN sets the delivery status notification list, only is set when SMTP server supports DSN extension
+//
+// To preserve the original recipient of an email message, for example, if it is forwarded to another address, set preserveOriginalRecipient to true
+func (email *Email) SetDSN(dsn []DSN, preserveOriginalRecipient bool) *Email {
+	if email.Error != nil {
+		return email
+	}
+
+	email.dsn = dsn
+	email.preserveOriginalRecipient = preserveOriginalRecipient
+
+	return email
+}
+
 // GetFrom returns the sender of the email, if any
 func (email *Email) GetFrom() string {
 	from := email.returnPath
@@ -710,6 +757,9 @@ func (email *Email) SendEnvelopeFrom(from string, client *SMTPClient) error {
 		msg = email.GetMessage()
 	}
 
+	client.dsn = email.dsn
+	client.preserveOriginalRecipient = email.preserveOriginalRecipient
+
 	return send(from, email.recipients, msg, client)
 }
 
@@ -864,10 +914,13 @@ func (server *SMTPServer) Connect() (*SMTPClient, error) {
 		}
 	}
 
+	_, hasDSN := c.ext["DSN"]
+
 	return &SMTPClient{
 		Client:      c,
 		KeepAlive:   server.KeepAlive,
 		SendTimeout: server.SendTimeout,
+		hasDSNExt:   hasDSN,
 	}, server.validateAuth(c)
 }
 
@@ -965,9 +1018,31 @@ func sendMailProcess(from string, to []string, msg string, c *SMTPClient) error
 		return err
 	}
 
+	var dsn string
+	var dsnSet bool
+
+	if c.hasDSNExt && len(c.dsn) > 0 {
+		dsn = " NOTIFY="
+		if hasNeverDSN(c.dsn) {
+			dsn += NEVER.String()
+		} else {
+			dsn += strings.Join(dsnToString(c.dsn), ",")
+		}
+
+		if c.preserveOriginalRecipient {
+			dsn += " ORCPT=rfc822;"
+		}
+
+		dsnSet = true
+	}
+
 	// Set the recipients
 	for _, address := range to {
-		if err := c.Client.rcpt(address); err != nil {
+		if dsnSet && c.preserveOriginalRecipient {
+			dsn += address
+		}
+
+		if err := c.Client.rcpt(address, dsn); err != nil {
 			return err
 		}
 	}
@@ -1001,3 +1076,20 @@ func checkKeepAlive(client *SMTPClient) {
 		client.Close()
 	}
 }
+
+func hasNeverDSN(dsnList []DSN) bool {
+	for i := range dsnList {
+		if dsnList[i] == NEVER {
+			return true
+		}
+	}
+	return false
+}
+
+func dsnToString(dsnList []DSN) []string {
+	dsnString := make([]string, len(dsnList))
+	for i := range dsnList {
+		dsnString[i] = dsnList[i].String()
+	}
+	return dsnString
+}

smtp.go

@@ -244,11 +244,11 @@ func (c *smtpClient) mail(from string, extArgs ...map[string]string) error {
 // rcpt issues a RCPT command to the server using the provided email address.
 // A call to Rcpt must be preceded by a call to Mail and may be followed by
 // a Data call or another Rcpt call.
-func (c *smtpClient) rcpt(to string) error {
+func (c *smtpClient) rcpt(to, dsn string) error {
 	if err := validateLine(to); err != nil {
 		return err
 	}
-	_, _, err := c.cmd(25, "RCPT TO:<%s>", to)
+	_, _, err := c.cmd(25, "RCPT TO:<%s>%s", to, dsn)
 	return err
 }
 

smtp_test.go

@@ -231,7 +231,7 @@ func TestBasic(t *testing.T) {
 		t.Fatalf("AUTH failed: %s", err)
 	}
 
-	if err := c.rcpt("[email protected]>\r\nDATA\r\nInjected message body\r\n.\r\nQUIT\r\n"); err == nil {
+	if err := c.rcpt("[email protected]>\r\nDATA\r\nInjected message body\r\n.\r\nQUIT\r\n", ""); err == nil {
 		t.Fatalf("RCPT should have failed due to a message injection attempt")
 	}
 	if err := c.mail("[email protected]>\r\nDATA\r\nAnother injected message body\r\n.\r\nQUIT\r\n"); err == nil {
@@ -240,7 +240,7 @@ func TestBasic(t *testing.T) {
 	if err := c.mail("[email protected]"); err != nil {
 		t.Fatalf("MAIL failed: %s", err)
 	}
-	if err := c.rcpt("[email protected]"); err != nil {
+	if err := c.rcpt("[email protected]", ""); err != nil {
 		t.Fatalf("RCPT failed: %s", err)
 	}
 	msg := `From: [email protected]
@@ -1025,8 +1025,9 @@ func init() {
 }
 
 // localhostCert is a PEM-encoded TLS cert generated from src/crypto/tls:
-// go run generate_cert.go --rsa-bits 1024 --host 127.0.0.1,::1,example.com \
-// 		--ca --start-date "Jan 1 00:00:00 1970" --duration=1000000h
+//
+//	go run generate_cert.go --rsa-bits 1024 --host 127.0.0.1,::1,example.com \
+//			--ca --start-date "Jan 1 00:00:00 1970" --duration=1000000h
 var localhostCert = []byte(`
 -----BEGIN CERTIFICATE-----
 MIICFDCCAX2gAwIBAgIRAK0xjnaPuNDSreeXb+z+0u4wDQYJKoZIhvcNAQELBQAw