Here's a suggestion for a filter:
/**
* Filters a comment's approval status before it is set.
*
* @since 2.1.0
* @since 4.9.0 Returning a WP_Error value from the filter will shortcircuit comment insertion and
* allow skipping further processing.
*
* @param bool|string|WP_Error $approved The approval status. Accepts 1, 0, 'spam' or WP_Error.
* @param array $commentdata Comment data.
*/
$approved = apply_filters( 'pre_comment_approved', $approved, $commentdata );
where we can e.g. try the email domain parser by Gabriel Livan:
add_filter( 'pre_comment_approved', function( $approved, $commentdata ) {
$domain = substr( strrchr( $commentdata['comment_author_email'], '@' ), 1 );
$banned_domains = [
'example.com',
'example.org',
'example.net',
'localhost'
];
if ( in_array( $domain, $banned_domains ) ) {
wp_die( __( 'Please use a real email!' ) );
//return new WP_Error( 'comment_real_email', __( 'Please use a real email!' ) );
}
return $approved;
}, 10, 2 );
where $banned_domains is the list of email domains we want to stop before it's saved into our database.
Another one that fires earlier is:
/**
* Filters a comment's data before it is sanitized and inserted into the database.
*
* @since 1.5.0
*
* @param array $commentdata Comment data.
*/
$commentdata = apply_filters( 'preprocess_comment', $commentdata );