ATtiny85 (Digispark) with 433mhz RF transmitter not working
I got it to work. The last comment of @Maximilian Gerhardt about I/O output got me thinking and I read up on it. So if I use 5V to power the ATtiny the I/O output voltage representation for 1 and 0 are significantly higher then with 3.3V. So now I underclocked the ATtiny to 8Mhz in order to use a 3.3V power supply. This way the I/O output matches the transmitters expectations. Now the communication is working. 

Thanks for your help Max!