diff --git a/phpBB/includes/functions_messenger.php b/phpBB/includes/functions_messenger.php
index b7f3eeda36..eafbb01cf9 100644
--- a/phpBB/includes/functions_messenger.php
+++ b/phpBB/includes/functions_messenger.php
@@ -1840,66 +1840,84 @@ class smtp_class
 }
 
 /**
-* Encodes the given string for proper display in UTF-8.
-*
-* This version is using base64 encoded data. The downside of this
-* is if the mail client does not understand this encoding the user
-* is basically doomed with an unreadable subject.
-*
-* Please note that this version fully supports RFC 2045 section 6.8.
-*
-* @param string $str
-* @param string $eol End of line we are using (optional to be backwards compatible)
-*/
+ * Encodes the given string for proper display in UTF-8 or US-ASCII.
+ *
+ * This version is based on iconv_mime_encode() implementation
+ * from symfomy/polyfill-iconv
+ * https://github.com/symfony/polyfill-iconv/blob/fd324208ec59a39ebe776e6e9ec5540ad4f40aaa/Iconv.php#L355
+ *
+ * @param string $str
+ * @param string $eol Lines delimiter (optional to be backwards compatible)
+ *
+ * @return string
+ */
 function mail_encode($str, $eol = "\r\n")
 {
 	// Check if string contains ASCII only characters
 	$is_ascii = strlen($str) === utf8_strlen($str);
 
-	// Define start delimimter, end delimiter and spacer
+	$scheme = $is_ascii ? 'Q' : 'B';
+
+	// Define start delimiter, end delimiter
 	// Use the Quoted-Printable encoding for ASCII strings to avoid unnecessary encoding in Base64
-	$start = $is_ascii ? "=?US-ASCII?Q?" : "=?UTF-8?B?";
-	$end = "?=";
-	$delimiter = "$eol ";
+	$start = '=?' . ($is_ascii ? 'US-ASCII' : 'UTF-8') . '?' . $scheme . '?';
+	$end = '?=';
 
 	// Maximum encoded-word length is 75 as per RFC 2047 section 2.
-	// $split_length *must* be a multiple of 4, but <= 75 - strlen($start . $delimiter . $end)!!!
-	$split_length = 75 - strlen($start . $delimiter . $end);
+	// $split_length *must* be a multiple of 4, but <= 75 - strlen($start . $eol . $end)!!!
+	$split_length = 75 - strlen($start . $eol . $end);
 	$split_length = $split_length - $split_length % 4;
 
-	// Use the Quoted-Printable encoding for ASCII strings to avoid unnecessary encoding in Base64
-	// quoted_printable_encode() splits lines at length of 75 characters with =\r\n delimiter, amend this feature
-	$encoded_str = $is_ascii ? str_replace("=\r\n", '', quoted_printable_encode($str)) : base64_encode($str);
+	$line_length = strlen($start) + strlen($end);
+	$line_offset = strlen($start) + 1;
+	$line_data = '';
 
-	// If encoded string meets the limits, we just return with the correct data.
-	if (strlen($encoded_str) <= $split_length)
+	$is_quoted_printable = 'Q' === $scheme;
+
+	preg_match_all('/./us', $str, $chars);
+	$chars = $chars[0] ?? [];
+
+	$str = [];
+	foreach ($chars as $char)
 	{
-		return $start . $encoded_str . $end;
-	}
+		$encoded_char = $is_quoted_printable
+			? $char = preg_replace_callback(
+				'/[=_\?\x20\x00-\x1F\x80-\xFF]/',
+				function ($matches)
+				{
+					$hex = dechex(ord($matches[0]));
+					$hex = strlen($hex) == 1 ? "0$hex" : $hex;
+					return '=' . strtoupper($hex);
+				},
+				$char
+			)
+			: base64_encode($line_data . $char);
 
-	// If there is only ASCII data, we just return what we want, correctly splitting the lines.
-	if ($is_ascii)
-	{
-		return $start . implode($end . $delimiter . $start, str_split($encoded_str, $split_length)) . $end;
-	}
-
-	// UTF-8 data, compose encoded lines
-	$array = utf8_str_split($str);
-	$str = '';
-
-	while (count($array))
-	{
-		$text = '';
-
-		while (count($array) && intval((strlen($text . $array[0]) + 2) / 3) << 2 <= $split_length)
+		if (isset($encoded_char[$split_length - $line_length]))
 		{
-			$text .= array_shift($array);
+			if (!$is_quoted_printable)
+			{
+				$line_data = base64_encode($line_data);
+			}
+			$str[] = $start . $line_data . $end;
+			$line_length = $line_offset;
+			$line_data = '';
 		}
 
-		$str .= $start . base64_encode($text) . $end . $delimiter;
+		$line_data .= $char;
+		$is_quoted_printable && $line_length += strlen($char);
 	}
 
-	return substr($str, 0, -strlen($delimiter));
+	if ($line_data !== '')
+	{
+		if (!$is_quoted_printable)
+		{
+			$line_data = base64_encode($line_data);
+		}
+		$str[] = $start . $line_data . $end;
+	}
+
+	return implode($eol . ' ', $str);
 }
 
 /**
diff --git a/tests/email/headers_encoding_test.php b/tests/email/headers_encoding_test.php
new file mode 100644
index 0000000000..a884a4cc21
--- /dev/null
+++ b/tests/email/headers_encoding_test.php
@@ -0,0 +1,48 @@
+<?php
+/**
+ *
+ * This file is part of the phpBB Forum Software package.
+ *
+ * @copyright (c) phpBB Limited <https://www.phpbb.com>
+ * @license GNU General Public License, version 2 (GPL-2.0)
+ *
+ * For full copyright and license information, please see
+ * the docs/CREDITS.txt file.
+ *
+ */
+
+class phpbb_headers_encoding_test extends phpbb_test_case
+{
+	protected function setUp(): void
+	{
+		global $phpbb_root_path, $phpEx;
+
+		if (!function_exists('mail_encode'))
+		{
+			include($phpbb_root_path . 'includes/functions_messenger.' . $phpEx);
+		}
+	}
+
+	public function headers_encoding_data()
+	{
+		return [
+			['test@yourdomain.com <phpBB fake test email>', 'Q', 'US-ASCII'],
+			['test@yourdomain.com <Несуществующий почтовый адрес phpBB>', 'B', 'UTF-8'],
+			["\xE3\x83\x86\xE3\x82\xB9\xE3\x83\x88\xE3\x83\x86\xE3\x82\xB9\xE3\x83\x88", 'B', 'UTF-8'],
+		];
+	}
+
+	/**
+	 * @dataProvider headers_encoding_data
+	 */
+	public function test_headers_encoding($header, $scheme, $encoding)
+	{
+		$encoded_string = mail_encode($header);
+		$this->assertStringStartsWith("=?$encoding?$scheme?", $encoded_string);
+		$this->assertStringEndsWith('?=', $encoded_string);
+
+		// Result of iconv_mime_decode() on decoded header should be equal to initial header
+		$decoded_string = iconv_mime_decode($encoded_string, 0, $encoding);
+		$this->assertEquals(0, strcmp($header, $decoded_string));
+	}
+}