I have it working at the moment by replacing: void sendToPushingBox(String devid) with void sendToPushingBox(char devid[]) although I think both should work fine. It also worked before with String, and when I send exactly the same text to serial, I se it 100% correct as it should be. Anyway it seems to work now. Reply from the server is OK as well.
I don't need to put all Serial.prints on 1 line, it works as the api-example as well. I have the impression that somewhere in Ethernet.client something is a bit unstable.
The issue I have now is that mails arrive only after several hours. I asked people from Pushingbox how this is possible. Before it was instantanious.