Now the command works correctly.
Thanks for the hints you gave me.
I will mark your previous answer as the resolution of my problem.
That is correct, even if I am not able to produce/write a custom template to change port 25 to 26 or to add port 26 as in @mmarkuz’s solution for NS7.
I am still confused in using NS8. It seems that it is not as simple as NS7, at least for me at the moment.
For now, I managed to install and it works “as is” the Mail, Nextcloud, MariaDB, NGiNX, WebTop, Collabora (still untested) modules.
In addition to the modification of SMTP port 25, I think I will have something to do with the operation of osTicket (it is not “native” for NGiNX but I found some tutorials for configuring NGiNX; I guess that your submissions to the NS8 dev manual will have to be taken into account for the solution. …).
The next step is to install WordPress and get my website up and running.
podman exec -ti postfix vi /etc/postfix/main.cf.d/myoverride.cf
I inserted the following parameter:
known_tcp_ports = lmtp=24, smtp=26, smtps=submissions=465, submission=587
systemctl --user reload postfix
I think it should work because after modifying main.cf with the new settings, I can no longer send/receive emails (with WebTop).
However, in NS8 mail I have nowhere to configure mail relay because this “submodule” does not exist. Relay exists in NS7 and through it mails are sent/received via PMG (I use this configuration now with NS7).