1) Have postfix running and confirm it works on startup
2) Install nfs-kernel-server
3) Reboot
4) Run "sudo service postfix", "sudo service portmap", "sudo service nfs-kernel-server" and check which services are running
5) If none of them are running, start them (postfix, portmap and nfs-kernel-server)
6) Remove nfs-kernel-server, reboot and see postfix running.
Steps to recreate the issue:
1) Have postfix running and confirm it works on startup
2) Install nfs-kernel-server
3) Reboot
4) Run "sudo service postfix", "sudo service portmap", "sudo service nfs-kernel-server" and check which services are running
5) If none of them are running, start them (postfix, portmap and nfs-kernel-server)
6) Remove nfs-kernel-server, reboot and see postfix running.