sudo less /etc/nullmailer/adminaddr
myMailAddress@gmx.de

sudo less /etc/nullmailer/remotes
mail.gmx.net smtp --port=465 --starttls --user=myMailProxy@gmx.de --pass=someSecret

....
root: myMailAddress@gmx.de

sudo service nullmailer restart

mail -s "whatever" root < /dev/null

Apr 20 22:20:58 desktop-pc nullmailer[4727]: smtp: Failed:
Apr 20 22:20:58 desktop-pc nullmailer[4717]: Sending failed:  Protocol error