3 files changed, 18 insertions, 0 deletions
diff --git a/src/mime/quotedprintable/reader_test.go b/src/mime/quotedprintable/reader_test.go
index c12d2ca3db..885d1a650d 100644
--- a/src/mime/quotedprintable/reader_test.go
+++ b/src/mime/quotedprintable/reader_test.go
@@ -70,6 +70,11 @@ func TestReader(t *testing.T) {
 		// Transport padding
 		{in: "foo= \r\nbar", want: "foobar"},
 		{in: "foo=\t \r\nbar", want: "foobar"},
+		{
+			// Line break with period at the end.
+			in:   strings.Repeat(".", 75) + "=\r\n=2E\r\n=2E\r\nfooter.",
+			want: strings.Repeat(".", 76) + "\r\n.\r\nfooter.",
+		},
 	}
 	for _, tt := range tests {
 		var buf strings.Builder
diff --git a/src/mime/quotedprintable/writer.go b/src/mime/quotedprintable/writer.go
index 69b5a11232..7fae1f08a9 100644
--- a/src/mime/quotedprintable/writer.go
+++ b/src/mime/quotedprintable/writer.go
@@ -133,6 +133,13 @@ func (w *Writer) checkLastByte() error {
 	}
 
 	b := w.line[w.i-1]
+	if w.i == 1 && b == '.' {
+		w.i--
+		if err := w.encode(b); err != nil {
+			return err
+		}
+		return nil
+	}
 	if isWhitespace(b) {
 		w.i--
 		if err := w.encode(b); err != nil {
diff --git a/src/mime/quotedprintable/writer_test.go b/src/mime/quotedprintable/writer_test.go
index 07411fe269..b53bb76d5e 100644
--- a/src/mime/quotedprintable/writer_test.go
+++ b/src/mime/quotedprintable/writer_test.go
@@ -88,6 +88,12 @@ func testWriter(t *testing.T, binary bool) {
 			in:   strings.Repeat(" ", 77),
 			want: strings.Repeat(" ", 75) + "=\r\n =20",
 		},
+		{
+			// Line break with period at the end.
+			in:    strings.Repeat(".", 76) + "\n.\nfooter.",
+			want:  strings.Repeat(".", 75) + "=\r\n=2E\r\n=2E\r\nfooter.",
+			wantB: strings.Repeat(".", 75) + "=\r\n.=0A.=0Afooter.",
+		},
 	}
 
 	for _, tt := range tests {