The server does not seem to support unique message numbers, but this is a requirement for leaving messages on the server. Since some servers do not correctly announce their capabilities you still have the possibility to turn leaving fetched messages on the server on.
Dizze tsjinner stipet gjin unike berjochtnûmers, mar dit is in fereaske by it efterlitten fan berjochten op de tsjinner. Omdat guon tsjinner har mooglikheden net goed oanjouwe kin it wêze dat jo dizze opsje dochs oansette kinne.