At last I’ve managed to make it working!
For my configuration tests I was using the following command:
sudo ./launcher destroy app && sudo ./launcher start app
I’ve finally executed the rebuild command
sudo ./launcher rebuild app
with the following configuration (all other non displayed DISCOURSE_SMTP_* parameter were commented)
DISCOURSE_SMTP_ADDRESS: my.smtp.server.tld
DISCOURSE_SMTP_AUTHENTICATION: none
DISCOURSE_SMTP_OPENSSL_VERIFY_MODE: none
and the mail was finally sent, so I could continue my installation!