Sorry, I completely forgot this thread. I found a solution.It wasn't the umlauts, I think. The Name of the recipient was set in quotationmarks - "Die studentischen Hilfskräfte" - and that got double-quoted - like ""Die studentischen Hilfskräfte"" - and it seems that caused the problem. I changed the Name in Users to Die studentischen Hilfskräfte without quotationmarks and it's working fine now.