Ever wondered why there are no new emails coming in from your shop contact form since the upgrade to Enterprise Edition 1.9.0.0? :) It's because each and every attempt to submit a contact form results in an error "Unable to submit your request. Please, try again later". The cause of this error is the newer version of Zend Framework libraries used, which are now prohibiting setting of "Reply-to" header in an old way Magento still tries to use.

The most quick and dirty fix for this issue is to comment out the line 97 in app/code/core/Mage/Contacts/controllers/IndexController.php

// ->setReplyTo($post['email'])

However, you may want this header and in this case you'll need to do some research. It could happen to be not enough to just fix the line 501 in app/code/core/Mage/Core/Model/Email/Template.php

 public function setReplyTo($email) {
// $this->getMail()->addHeader('Reply-To', $email); // <-- change this line $this->getMail()->setReplyTo($email); // <-- to read this return $this; }

due to the Zend_Mail class now also prohibits setting of this header more than once and you might end up with some other emails not being sent (if anywhere else in the code there is an attempt to re-set this header).