I got a solution to send auto-responses in HTML and also send text with spelling accents
cp /etc/postfix/vacation_class.php /etc/postfix/vacation_class.bak
nano /etc/postfix/vacation_class.php
Before:
$content_type = property_exists($this->reply,'ishtml') && $this->reply->ishtml == 1 ? 'text/html' : 'text/plain';
$headers .= 'Content-type: ' . $content_type . '; charset=' . $this->reply->charset . "\r\n";
After
$content_type = property_exists($this->reply,'ishtml') && $this->reply->ishtml == 1 ? 'text/html' : 'text/html';
$headers .= 'Content-type: ' . $content_type . '; charset=utf-8\r\n';
This is just an adaptation! The auto-response feature needs some attention from the developers, It has a lot of bugs...