The server does not seem to support unique message numbers, but this is a requirement for leaving messages on the server; therefore, this option has been disabled. 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, dit is in fereaske by it efterlitten fan berjochten op de tsjinner. Dizze opsje is dêrom útskeakele. Omdat guon tsjinners har mooglikheden net goed oanjouwe kin it wêze dat jo dizze opsje dochs oansette kinne.