Sending email : "Error can not use ipv4 addresses with current network"

Removed IPv6. Same error.

$ sudo nano /etc/sysctl .conf

Add this to the end:

net.ipv6.conf.all.disable_ipv6=1

net.ipv6.conf.default.disable_ipv6=1

net.ipv6.conf.lo.disable_ipv6=1

net.ipv6.conf.eth0.disable_ipv6 = 1

rebooted

ifconfig now shows:

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.60.20 netmask 255.255.255.0 broadcast 192.168.60.255
ether dc:a6:32:79:53:40 txqueuelen 1000 (Ethernet)
RX packets 20418 bytes 2811277 (2.6 MiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 25720 bytes 9399504 (8.9 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0