Fixed Plugin

This should do the trick:

/**
	 * Modifies the emails by replacing the conditionals [if field_name][/if]
	 *
	 * @param [Object] $cf7 the contanct form 7 object
	 * @return void
	 * @author Tobias Braner
	 **/
	public function before_send_mail($contact_form) {
  		$submission = WPCF7_Submission::get_instance();
		if ( $submission ) {
			$posted_data = $submission->get_posted_data();
			$mail = $contact_form->prop('mail');
			$mail2 = $contact_form->prop('mail2');
			$mail_body = $mail['body'];
			$mail2_body = $mail2['body'];
			$mail['body'] = $this->process_conditions($mail_body, $posted_data);
			$mail2['body'] = $this->process_conditions($mail2_body, $posted_data);
			$contact_form->set_properties(array('mail' => $mail, 'mail2' => $mail2));
		}
	}

	/**
	 * Takes the wpcf7 object and replaces all conditions.
	 *
	 * @return void
	 * @author Tobias Braner
	 **/
	private function process_conditions($mail_body, $posted_data) {
		$updated_email_body = $mail_body;
		$matches = array();
		$num_matches = preg_match_all($this->regexp, $mail_body, $matches);
		for ($i=0; $i < $num_matches; $i++) {
			$expression = $matches[WPCF7MC_EXPRESSION][$i];
			$variable = $matches[WPCF7MC_VARIABLE][$i];
			$value = $matches[WPCF7MC_VALUE][$i];
			if (empty($posted_data[$variable][0]) and array_key_exists($variable, $posted_data)) {
				$updated_email_body = str_replace($expression, '', $updated_email_body);
			} else {
				$updated_email_body = str_replace($expression, $value, $updated_email_body);
			}
		}
		return $updated_email_body;
	}

There is just one very short file in the plugin folder (besides the readme.txt). Open it and replace the functions inside with the functions I sent you. If you need further assistance, my email address is my wordpress username at gmail com.