#166: Add delivery status indication for messages

This update introduces a new property, `isDelivered`, in the message struct to track if a message has been successfully sent or not. Additionally, this also includes a new helper function `IsDelivered` to easily retrieve the status. This feature enhances the tracking capabilities of outgoing messages across different client files.

client_119.go

@@ -91,6 +91,7 @@ func (c *Client) Send(ml ...*Msg) error {
 			errs = append(errs, se)
 			continue
 		}
+		m.isDelivered = true
 
 		if err := w.Close(); err != nil {
 			se := &SendError{Reason: ErrSMTPDataClose, errlist: []error{err}, isTemp: isTempError(err)}

client_120.go

@@ -88,6 +88,7 @@ func (c *Client) Send(ml ...*Msg) (rerr error) {
 			rerr = errors.Join(rerr, m.sendError)
 			continue
 		}
+		m.isDelivered = true
 
 		if err := w.Close(); err != nil {
 			m.sendError = &SendError{Reason: ErrSMTPDataClose, errlist: []error{err}, isTemp: isTempError(err)}

msg.go

@@ -94,6 +94,9 @@ type Msg struct {
 	// genHeader is a slice of strings that the different generic mail Header fields
 	genHeader map[Header][]string
 
+	// isDelivered signals if a message has been delivered or not
+	isDelivered bool
+
 	// middlewares is the list of middlewares to apply to the Msg before sending in FIFO order
 	middlewares []Middleware
 
@@ -507,6 +510,11 @@ func (m *Msg) SetUserAgent(a string) {
 	m.SetGenHeader(HeaderXMailer, a)
 }
 
+// IsDelivered will return true if the Msg has been successfully delivered
+func (m *Msg) IsDelivered() bool {
+	return m.isDelivered
+}
+
 // RequestMDNTo adds the Disposition-Notification-To header to request a MDN from the receiving end
 // as described in RFC8098. It allows to provide a list recipient addresses.
 // Address validation is performed