Already using email address validation: ./pages/modules/recommend.php (it's further down the code)

feedback.php was only using email for a comcode tag, so instead of warn_exit, I simply made it not use the email comcode tag if the email is invalid.

For all other backend cases, to avoid lots of repeating code, I added a new INPUT_FILTER_EMAIL_ADDRESS bitmask which can be used on post_param_string for email addresses.