ZP-1377 Fix undefined index warnings in imap backend.

Released under the Affero GNU General Public License (AGPL) version 3.

src/backend/imap/imap.php

@@ -205,8 +205,12 @@ class BackendIMAP extends BackendDiff implements ISearchProvider {
             ZLog::Write(LOGLEVEL_DEBUG, sprintf("BackendIMAP->SendMail(): To defined: %s", $toaddr));
         }
 
+        if (isset($message->headers["to"])) {
             $message->headers["to"] = Utils::CheckAndFixEncodingInHeadersOfSentMail($Mail_RFC822->parseAddressList($message->headers["to"]));
+        }
+        if (isset($message->headers["cc"])) {
             $message->headers["cc"] = Utils::CheckAndFixEncodingInHeadersOfSentMail($Mail_RFC822->parseAddressList($message->headers["cc"]));
+        }
 
         unset($Mail_RFC822);