This issue needs more informations.

For example,
encoding of your string. it is ISO-8859-1 ? I do not know what language you use.
use or not setHeaderEncoding() .

Dear Satoru Yoshida,

Here the answers to your questions:

The encoding is standard (ISO-8859-1)
The language is Danish (but why is that important, it is about the character string I think)
setHeaderEncoding isn't used, see the php code below:

This is the code when the problem occurs:

<?php
require_once('Zend/Mail.php');

$mail = new Zend_Mail();

$mail->setSubject('Bekræftigelse af reservation med Goopoint Online Appointments den 16-04-2009 kl. 09:00.');
$mail->setBodyText('Test');

$mail->addTo('info@spintop.nl');
$mail->send();
?>

Committed Fix to trunk.

Changed Zend_Mime::encodeQuotedPrintableHeader() again to only break at spaces. This is the only possible way to ensure in every setup that headers are displayed correctly in many different mail-clients when they are broken into several lines.

If problems occur with too long headers, because no spaces occur please use Base64 Encoding, which does not suffer from this problems.