Found out just now what the problem was.

I am writing this to help others: the federation was active in that instance but it was also marked as a private instance. This gave no problems in using it until the update (probably because it is restarted and re-run Docker).

I removed the federation from the config file leaving only the private instance and it restarted immediately.

yes, actually i tried ‘none’, ‘tls’ and ‘starttls’ but with no success. I tried with external SMTP because the default created by ansible does not work:

  email: {
    smtp_server: "postfix:25"
    smtp_from_address: "noreply@mydomain.it"
    tls_type: "none"
  }

It is the same thing that I thought. Unfortunately, if I try to manually force the port by writing smtps.aruba.it:465 as I said the server freezes for several minutes, goes to 504 and does not send any email.

So I can not understand if the port must be entered in another way (?) or if I have to install something on the server first to be able to send an email through an external SMTP as in this case?