Is a form on a regular domain (http://www.domainname.com/form.php) with the following action tag:
<form action="https://www.domainname.com/mail.php" method="POST">
indeed secure?
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No.
The form can be intercepted on its way to the user (a man in the middle attack) and the page modified so that the data will be submitted somewhere else (possibly with JavaScript so the data theft is harder to spot).
The request to the HTTPS site will be secure, but not in time to avoid the data being intercepted.
Also, the page containing the form will not be marked as secure, which will not illicit trust from the user.