I'm sending an e-mail, and it gets data from the previous page via post.

When I get the email, any quotes in the message are escaped(\"). How can I fix this?

stripslashes
http://us2.php.net/manual/en/function.stripslashes.php

you have two ways to go.
one is to disable magic_quotes in php.ini

magic_quotes_gpc = Off
magic_quotes_runtime = Off
magic_quotes_sybase = Off

so it may be not the best way to go

otherway is to take out slashes from message body and subject
ex:

PHP Code:

$body=stripslashes($body);
$subject=stripslashes($subject);