Email - SMTP TLS 1.3 error (TLS 1.2 OK)

I have installed my NC instance on a VPS, running in docker with an NGINX reverse proxy in front and on the VPS an email server up and running

Configuring Nextcloud email server fails to test the parameters (same than in another email app Roundcube)

When from inside the container i try to send an email by a direct connection to the SMTP server it works only if i force TLS 1.2

openssl s_client -starttls -tls1_2 smtp -connect

I can then send the multiple commans (EHLO, AUTH, etc) successfully
By default it uses TLS 1.3 and the error comes from the renegociation command :

read R BLOCK
40E78503847F0000:error:0A00010A:SSL routines:can_renegotiate:wrong ssl version:../ssl/ssl_lib.c:2304:

I tried to add this to config.php but with no positive result :

"mail_smtpstreamoptions" => array(
    'ssl' => array(
        'allow_self_signed' => true,
        'verify_peer' => false,
        'verify_peer_name' => false

Any idea how to make NC to support TLS 1.3 or to force TLS 1.2 in the SMTP connection ?