Here’s how to fix the following Postfix issue:

send-mail: warning: inet_protocols: IPv6 support is disabled: Address family not supported by protocol
send-mail: warning: inet_protocols: configuring for IPv4 support only
postdrop: warning: inet_protocols: IPv6 support is disabled: Address family not supported by protocol
postdrop: warning: inet_protocols: configuring for IPv4 support only

# /usr/sbin/postconf | grep inet_protocols
inet_protocols = all

# vi /etc/postfix/main.cf

Search for: inet_protocols and change it from “inet_protocols = all” to “inet_protocols = ipv4”
Save /etc/postfix/main.cf and and restart Postfix service:

# /etc/init.d/postfix restart