$singleByteCharacters = str_split('abcdefghijklmnopqrstuvwxyz ');
$multiByteCharacters = array('…', 'ü', 'ö', 'ä');

$imePrefs = array(
    'scheme' => 'Q',
    'input-charset'  => 'utf-8',
    'output-charset' => 'utf-8',
    'line-length' => 74,
    'line-break-chars' => "\r\n",
);

$errorPositions = array();

for ($strlen = 1; $strlen < 200; $strlen++) {
    for ($i = 0; $i < 1000; $i++) {
        $subject = '';
        $multiBytePosition = rand(1, $strlen);
        for ($j = 0; $j < $strlen; $j++) {
            if ($j == $multiBytePosition) {
                $array = &$multiByteCharacters;
            } else {
                $array = &$singleByteCharacters;
            }
            $index = array_rand($array);
            $subject .= $array[$index];
        }
        $subject = substr($subject, 1);
        if (!@iconv_mime_encode('subject', $subject, $imePrefs)) {
            if (!isset($errorPositions[$multiBytePosition])) {
                $errorPositions[$multiBytePosition] = 0;
            }
            $errorPositions[$multiBytePosition]++;
        }
    }
}

ksort($errorPositions);
foreach ($errorPositions as $position => $errorCount) {
    echo "$position: $errorCount\n";
}

# example output:
# 23: 161
# 24: 238
# 25: 846
# 49: 1
# 50: 19
# 51: 47
# 52: 144
# 53: 264
# 54: 299
# 55: 119
# 78: 3
# 79: 5
# 80: 26
# 81: 44
# 82: 58
# 83: 111
# 84: 141
# 85: 149
# 86: 50
# 107: 1
# 108: 2
# 109: 4
# 110: 17
# 111: 22
# 112: 46
# 113: 60
# 114: 72
# 115: 72
# 116: 62
# 117: 32
# 138: 3
# 139: 1
# 140: 8
# 141: 15
# 142: 23
# 143: 31
# 144: 30
# 145: 23
# 146: 27
# 147: 21
# 148: 8
# 166: 1
# 168: 2
# 169: 1
# 170: 3
# 171: 5
# 172: 16
# 173: 10
# 174: 16
# 175: 9
# 176: 5
# 177: 15
# 178: 5
# 179: 1