The server does not seem to support fetching message headers, but this is a requirement for filtering messages on the server. Since some servers do not correctly announce their capabilities you still have the possibility to turn filtering messages on the server on.
Mei dizze tsjinner kinne jo gjin berjochtkoppen ophelje, wat wol in fereaske is om berjochten op de tsjinner te filterjen. Omdat guon tsjinner har mooglikheden net goed oanjouwe kin it wêze dat jo dizze opsje dochs oansette kinne.