diff --git a/internal/notification/const.go b/internal/notification/const.go
index aee69c680..3534ee359 100644
--- a/internal/notification/const.go
+++ b/internal/notification/const.go
@@ -40,6 +40,10 @@ const (
 	fmtSMTPDialError    = "error dialing the SMTP server: %w"
 )
 
-var (
-	rfc2822DoubleNewLine = []byte("\r\n\r\n")
+const (
+	rfc2822NewLine = "\r\n"
+)
+
+var (
+	rfc2822DoubleNewLine = []byte(rfc2822NewLine + rfc2822NewLine)
 )
diff --git a/internal/notification/smtp_util_test.go b/internal/notification/smtp_util_test.go
new file mode 100644
index 000000000..62c33fd09
--- /dev/null
+++ b/internal/notification/smtp_util_test.go
@@ -0,0 +1,97 @@
+package notification
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/authelia/authelia/v4/internal/utils"
+)
+
+func TestNewMIMECharacteristics(t *testing.T) {
+	testCases := []struct {
+		name     string
+		expected MIMECharacteristics
+		have     []byte
+	}{
+		{
+			"ShouldDetectMessageCharacteristics7Bit",
+			MIMECharacteristics{},
+			createMIMEBytes(false, true, 5, 150),
+		},
+		{
+			"ShouldDetectMessageCharacteristicsLongLine",
+			MIMECharacteristics{LongLines: true},
+			createMIMEBytes(false, true, 3, 1200),
+		},
+		{
+			"ShouldDetectMessageCharacteristicsLF",
+			MIMECharacteristics{LineFeeds: true},
+			createMIMEBytes(false, false, 5, 150),
+		},
+		{
+			"ShouldDetectMessageCharacteristics8Bit",
+			MIMECharacteristics{Characters8BIT: true},
+			createMIMEBytes(true, true, 3, 150),
+		},
+		{
+			"ShouldDetectMessageCharacteristicsLongLineAndLF",
+			MIMECharacteristics{true, true, false},
+			createMIMEBytes(false, false, 3, 1200),
+		},
+		{
+			"ShouldDetectMessageCharacteristicsLongLineAnd8Bit",
+			MIMECharacteristics{true, false, true},
+			createMIMEBytes(true, true, 3, 1200),
+		},
+		{
+			"ShouldDetectMessageCharacteristics8BitAndLF",
+			MIMECharacteristics{false, true, true},
+			createMIMEBytes(true, false, 3, 150),
+		},
+		{
+			"ShouldDetectMessageCharacteristicsAll",
+			MIMECharacteristics{true, true, true},
+			createMIMEBytes(true, false, 3, 1200),
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			actual := NewMIMECharacteristics(tc.have)
+
+			assert.Equal(t, tc.expected, actual)
+		})
+	}
+}
+
+func createMIMEBytes(include8bit, crlf bool, lines, length int) []byte {
+	buf := &bytes.Buffer{}
+
+	for i := 0; i < lines; i++ {
+		for j := 0; j < length/100; j++ {
+			switch {
+			case include8bit:
+				buf.Write(utils.RandomBytes(50, utils.AlphaNumericCharacters, false))
+				buf.Write([]byte("£"))
+				buf.Write(utils.RandomBytes(49, utils.AlphaNumericCharacters, false))
+			default:
+				buf.Write(utils.RandomBytes(100, utils.AlphaNumericCharacters, false))
+			}
+		}
+
+		if n := length % 100; n != 0 {
+			buf.Write(utils.RandomBytes(n, utils.AlphaNumericCharacters, false))
+		}
+
+		switch {
+		case crlf:
+			buf.Write([]byte(rfc2822NewLine))
+		default:
+			buf.Write([]byte("\n"))
+		}
+	}
+
+	return buf.Bytes()
+}