How to hook into Contact Form 7 Before Send

I had to do this to prevent Email from being sent. Hope it helps.

/*
    Prevent the email sending step for specific form
*/
add_action("wpcf7_before_send_mail", "wpcf7_do_something_else");  
function wpcf7_do_something_else($cf7) {
    // get the contact form object
    $wpcf = WPCF7_ContactForm::get_current();

    // if you wanna check the ID of the Form $wpcf->id

    if (/*Perform check here*/) {
        // If you want to skip mailing the data, you can do it...  
        $wpcf->skip_mail = true;    
    }

    return $wpcf;
}

This code assumes that you are running the latest version of CF7 your code above used to work until a couple months ago when they went and did some refactoring of the code. [Apr 28 '15]

Since WPCF7 5.2 the wpcf7_before_send_mail hook has changed quite a lot. For reference, here is how to work with this hook in 5.2+

Skip mail

function my_skip_mail() {
    return true; // true skips sending the email
}
add_filter('wpcf7_skip_mail','my_skip_mail');

Or add skip_mail to the Additional Settings tab on your form in the admin area.

Getting the Form ID or Post ID

function wpcf7_before_send_mail_function( $contact_form, $abort, $submission ) {

    $post_id = $submission->get_meta('container_post_id');
    $form_id = $contact_form->id();

    // do something       

    return $contact_form;
    
}
add_filter( 'wpcf7_before_send_mail', 'wpcf7_before_send_mail_function', 10, 3 );

Get user inputted data

function wpcf7_before_send_mail_function( $contact_form, $abort, $submission ) {

    $your_name = $submission->get_posted_data('your-field-name');

    // do something       

    return $contact_form;
    
}
add_filter( 'wpcf7_before_send_mail', 'wpcf7_before_send_mail_function', 10, 3 );

Send the email to a dynamic recipient

function wpcf7_before_send_mail_function( $contact_form, $abort, $submission ) {

    $dynamic_email = '[email protected]'; // get your email address...

    $properties = $contact_form->get_properties();
    $properties['mail']['recipient'] = $dynamic_email;
    $contact_form->set_properties($properties);

    return $contact_form;
    
}
add_filter( 'wpcf7_before_send_mail', 'wpcf7_before_send_mail_function', 10, 3 );

I'd like to add that you could just use the wpcf7_skip_mail filter:

add_filter( 'wpcf7_skip_mail', 'maybe_skip_mail', 10, 2 );

function maybe_skip_mail( $skip_mail, $contact_form ) {

    if( /* your condition */ )
        $skip_mail = true;

    return $skip_mail;

}