Validate mail forwarding destination

interface/web/mail/lib/lang/de_mail_forward.lng

@@ -6,6 +6,8 @@ $wb['limit_mailforward_txt'] = 'Die maximale Anzahl an E-Mail Weiterleitungen f
 $wb['duplicate_mailbox_txt'] = 'Es existiert bereits ein E-Mail Konto mit dieser Adresse.';
 $wb['domain_txt'] = 'Domain';
 $wb['source_txt'] = 'Quell E-Mail Adresse';
+$wb['destination_error_empty'] = 'Das Weiterleitungsziel darf nicht leer sein.';
+$wb['destination_error_isemail'] = 'Das Weiterleitungsziel enthält mindestens eine ungültige E-Mail-Adresse.';
 $wb['email_error_isemail'] = 'Bitte geben Sie eine gültige E-Mail Adresse an.';
 $wb['send_as_txt'] = 'Senden als';
 $wb['send_as_exp'] = 'Ziel erlauben, die Adresse als Absender zu nutzen (Nur, falls das Ziel intern ist)';

interface/web/mail/lib/lang/en_mail_forward.lng

@@ -6,6 +6,8 @@ $wb['limit_mailforward_txt'] = 'The max. number of email forwarders for your acc
 $wb['duplicate_mailbox_txt'] = 'There is already a mailbox with this email address';
 $wb['domain_txt'] = 'Domain';
 $wb['source_txt'] = 'Source Email';
+$wb['destination_error_empty'] = 'The destination must not be empty.';
+$wb['destination_error_isemail'] = 'The destination contains at least one invalid email address.';
 $wb['email_error_isemail'] = 'Please enter a valid email address.';
 $wb['send_as_txt'] = 'Send as';
 $wb['send_as_exp'] = 'Allow target to send mail using this address as origin (if target is internal)';

interface/web/mail/mail_forward_edit.php

@@ -120,6 +120,17 @@ class page_action extends tform_actions {
 		unset($this->dataRecord["email_local_part"]);
 		unset($this->dataRecord["email_domain"]);
 
+		if(trim($this->dataRecord['destination']) == '') {
+			$app->tform->errorMessage .= $app->tform->lng('destination_error_empty') . '<br />';
+		} else {
+			$targets = preg_split('/\s*[,;]\s*/', trim($this->dataRecord['destination']));
+			foreach($targets as $target) {
+				if(!$target || filter_var($target, FILTER_VALIDATE_EMAIL) === false) {
+					$app->tform->errorMessage .= $app->tform->lng('destination_error_isemail') . '<br />'
+				}
+			}
+		}
+
 		//* Check if there is no active mailbox with this address
 		$tmp = $app->db->queryOneRecord("SELECT count(mailuser_id) as number FROM mail_user WHERE postfix = 'y' AND email = ?", $this->dataRecord["source"]);
 		if($tmp['number'] > 0) $app->tform->errorMessage .= $app->tform->lng("duplicate_mailbox_txt")."<br>";