diff --git a/smtp/data.go b/smtp/data.go
index c3f293b..7a59d75 100644
--- a/smtp/data.go
+++ b/smtp/data.go
@@ -40,8 +40,10 @@ func DataWrite(w io.Writer, r io.Reader) error {
 					c := p[n]
 					if c == '\n' {
 						if firstcr < 0 {
-							// Bare newline.
-							return ErrCRLF
+							if n > 0 || last != '\r' {
+								// Bare newline.
+								return ErrCRLF
+							}
 						} else if firstcr != n-1 {
 							// Bare carriage return.
 							return ErrCRLF
diff --git a/smtp/data_test.go b/smtp/data_test.go
index ac0496c..f438b08 100644
--- a/smtp/data_test.go
+++ b/smtp/data_test.go
@@ -129,3 +129,27 @@ func TestDataReader(t *testing.T) {
 		t.Fatalf("got err %v, expected io.ErrUnexpectedEOF", err)
 	}
 }
+
+func TestDataWriteLineBoundaries(t *testing.T) {
+	const valid = "Subject: test\r\n\r\nbody\r\n"
+	if err := DataWrite(io.Discard, &oneReader{[]byte(valid)}); err != nil {
+		t.Fatalf("data write: %v", err)
+	}
+}
+
+// oneReader returns data one byte at a time.
+type oneReader struct {
+	buf []byte
+}
+
+func (r *oneReader) Read(buf []byte) (int, error) {
+	if len(r.buf) == 0 {
+		return 0, io.EOF
+	}
+	if len(buf) == 0 {
+		return 0, nil
+	}
+	buf[0] = r.buf[0]
+	r.buf = r.buf[1:]
+	return 1, nil
+}