Bon à savoir: synology n'utilise que la methode LOGIN pour s'authentifier au SMTP (dovecot via postfix). ne pas oublier de rajouter la mention "login" a la ligne auth_mechanisms ...

Authentication didn't work. Synology uses the LOGIN method, where all other clients that I use are using the PLAIN method. I deliverately disabled LOGIN because the only clients that I could think of was Outlook Express, not really a client I would use in the next few hundred years. So, in order to make authentication work, LOGIN needs to be enabled. Since Postfix queries Dovecot for authentication, I had to adapt the Dovecot configuration.