round cube will be connecting on a local ip ie localhost(127.0.0.1),
How are you connecting from Andriod ie pop, smtp, activesync ect?
I would say that when you are connecting from Thunderbird you are using smtp and this is subject to the ip reputation checks,
You could test this by telnet'ing to your server from the client machine on port 25 or 587
telnet xx.xx.xx.xx.xx 25
You could edit the postfix main.conf and add your IP to
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128, xx.xx.xx.xx