Implement a proper io.Reader interface since the current implementation is flawed

msg.go

@@ -807,6 +807,26 @@ func (m *Msg) WriteToSendmailWithContext(ctx context.Context, sp string, a ...st
 	return nil
 }
 
+// NewReader returns a Msg reader that satisfies the io.Reader interface.
+// **Please note:** when creating a new reader, the current state of the Msg is taken, as
+// basis for the reader. If you perform changes on Msg after creating the reader, you need
+// to perform a call to Msg.UpdateReader first
+func (m *Msg) NewReader() io.Reader {
+	wbuf := bytes.Buffer{}
+	_, _ = m.Write(&wbuf)
+	r := &reader{buf: wbuf.Bytes()}
+	return r
+}
+
+// UpdateReader will update a reader with the content of the current Msg and reset the reader position
+// to the start
+func (m *Msg) UpdateReader(r *reader) {
+	wbuf := bytes.Buffer{}
+	_, _ = m.Write(&wbuf)
+	r.Reset()
+	r.buf = wbuf.Bytes()
+}
+
 // Read outputs the length of p into p to satisfy the io.Reader interface
 func (m *Msg) Read(p []byte) (int, error) {
 	wbuf := bytes.Buffer{}

reader.go

@@ -0,0 +1,33 @@
+package mail
+
+import "io"
+
+// reader is a type that implements the io.Reader interface for a Msg
+type reader struct {
+	buf []byte // contents are the bytes buf[off : len(buf)]
+	off int    // read at &buf[off], write at &buf[len(buf)]
+}
+
+// Read reads the length of p of the Msg buffer to satisfy the io.Reader interface
+func (r *reader) Read(p []byte) (n int, err error) {
+	if r.empty() {
+		r.Reset()
+		if len(p) == 0 {
+			return 0, nil
+		}
+		return 0, io.EOF
+	}
+	n = copy(p, r.buf[r.off:])
+	r.off += n
+	return n, nil
+}
+
+// Reset resets the Reader buffer to be empty, but it retains the underlying storage
+// for use by future writes.
+func (r *reader) Reset() {
+	r.buf = r.buf[:0]
+	r.off = 0
+}
+
+// empty reports whether the unread portion of the Reader buffer is empty.
+func (r *reader) empty() bool { return len(r.buf) <= r.off }