Revert of "Use PHP's own email validation routines"

This partially reverts commit 4235a08
which introduced a regression.

The PHP email validation rejects valid e-mail addresses with the form
user@hostname [1]. The PHPMailer accepts these, and furthermore it makes
sense to rely on the validation routine provided by the library we use
to send our emails.

[1] https://bugs.php.net/bug.php?id=49576

core/email_api.php

@@ -122,9 +122,9 @@ function email_is_valid( $p_email ) {
 		return true;
 	}
 
-	// check email address is a valid format
-	$t_email = filter_var($p_email, FILTER_SANITIZE_EMAIL);
-	if (filter_var($t_email, FILTER_VALIDATE_EMAIL)) {
+	# check email address is a valid format
+	$t_email = filter_var( $p_email, FILTER_SANITIZE_EMAIL );
+	if( PHPMailer::ValidateAddress( $t_email ) ) {
 		$t_domain = end( explode( '@', $t_email ) );
 
 		# see if we're limited to a set of known domains