diff --git a/msgwriter_test.go b/msgwriter_test.go
index a3d2e56..31aae21 100644
--- a/msgwriter_test.go
+++ b/msgwriter_test.go
@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"io"
 	"mime"
+	"os"
 	"runtime"
 	"strings"
 	"testing"
@@ -304,6 +305,81 @@ func TestMsgWriter_addFiles(t *testing.T) {
 		charset: CharsetUTF8,
 		encoder: getEncoder(EncodingQP),
 	}
+	tests := []struct {
+		name     string
+		filename string
+		expect   string
+	}{
+		{"normal US-ASCII filename", "test.txt", "test.txt"},
+		{"normal US-ASCII filename with space", "test file.txt", "test file.txt"},
+		{"filename with new lines", "test\r\n.txt", "test__.txt"},
+		{"filename with disallowed character:\x22", "test\x22.txt", "test_.txt"},
+		{"filename with disallowed character:\x2f", "test\x2f.txt", "test_.txt"},
+		{"filename with disallowed character:\x3a", "test\x3a.txt", "test_.txt"},
+		{"filename with disallowed character:\x3c", "test\x3c.txt", "test_.txt"},
+		{"filename with disallowed character:\x3e", "test\x3e.txt", "test_.txt"},
+		{"filename with disallowed character:\x3f", "test\x3f.txt", "test_.txt"},
+		{"filename with disallowed character:\x5c", "test\x5c.txt", "test_.txt"},
+		{"filename with disallowed character:\x7c", "test\x7c.txt", "test_.txt"},
+		{"filename with disallowed character:\x7f", "test\x7f.txt", "test_.txt"},
+		{
+			"japanese characters filename", "添付ファイル.txt",
+			"=?UTF-8?q?=E6=B7=BB=E4=BB=98=E3=83=95=E3=82=A1=E3=82=A4=E3=83=AB.txt?=",
+		},
+		{
+			"simplified chinese characters filename", "测试附件文件.txt",
+			"=?UTF-8?q?=E6=B5=8B=E8=AF=95=E9=99=84=E4=BB=B6=E6=96=87=E4=BB=B6.txt?=",
+		},
+		{
+			"cyrillic characters filename", "Тестовый прикрепленный файл.txt",
+			"=?UTF-8?q?=D0=A2=D0=B5=D1=81=D1=82=D0=BE=D0=B2=D1=8B=D0=B9_=D0=BF=D1=80?= " +
+				"=?UTF-8?q?=D0=B8=D0=BA=D1=80=D0=B5=D0=BF=D0=BB=D0=B5=D0=BD=D0=BD=D1=8B?= " +
+				"=?UTF-8?q?=D0=B9_=D1=84=D0=B0=D0=B9=D0=BB.txt?=",
+		},
+	}
+	for _, tt := range tests {
+		t.Run("addFile with filename sanitization: "+tt.name, func(t *testing.T) {
+			buffer := bytes.NewBuffer(nil)
+			msgwriter.writer = buffer
+			message := testMessage(t)
+			tmpfile, err := os.CreateTemp("", "attachment.*.tmp")
+			if err != nil {
+				t.Fatalf("failed to create tempfile: %s", err)
+			}
+			t.Cleanup(func() {
+				if err = os.Remove(tmpfile.Name()); err != nil {
+					t.Errorf("failed to remove tempfile: %s", err)
+				}
+			})
+
+			source, err := os.Open("testdata/attachment.txt")
+			if err != nil {
+				t.Fatalf("failed to open source file: %s", err)
+			}
+			if _, err = io.Copy(tmpfile, source); err != nil {
+				t.Fatalf("failed to copy source file: %s", err)
+			}
+			if err = tmpfile.Close(); err != nil {
+				t.Fatalf("failed to close tempfile: %s", err)
+			}
+			if err = source.Close(); err != nil {
+				t.Fatalf("failed to close source file: %s", err)
+			}
+			message.AttachFile(tmpfile.Name(), WithFileName(tt.filename))
+			msgwriter.writeMsg(message)
+			if msgwriter.err != nil {
+				t.Errorf("msgWriter failed to write: %s", msgwriter.err)
+			}
+			ctExpect := fmt.Sprintf(`Content-Type: text/plain; charset=utf-8; name="%s"`, tt.expect)
+			cdExpect := fmt.Sprintf(`Content-Disposition: attachment; filename="%s"`, tt.expect)
+			if !strings.Contains(buffer.String(), ctExpect) {
+				t.Errorf("expected content-type: %q, got: %q", ctExpect, buffer.String())
+			}
+			if !strings.Contains(buffer.String(), cdExpect) {
+				t.Errorf("expected content-disposition: %q, got: %q", cdExpect, buffer.String())
+			}
+		})
+	}
 	t.Run("message with a single file attached", func(t *testing.T) {
 		buffer := bytes.NewBuffer(nil)
 		msgwriter.writer = buffer