It turned out that one of my VPS had a problem during the vacation left. Just realized when doing a quick maintenance and check the log. There is one error with a fatal message: parameter inet_interfaces: no local interface found for :: 1.
And this error message is related to Postfix trying to send email via IPv6, which I can but I do not use VPS.
The solution? Easy really.
Edit Postfix configuration:
Customize the lines of the code below to exclusive use of IPv4 only:
# Enable IPv4, and IPv6 if supported inet_interfaces: ipv4