Turns out I need to fall back to the old postfix configuration with sasl_passwd support.
After set this in the main.cf postfix file:
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
mydestination = $myhostname, localhost.$mydomain, localhost
relayhost = [smtp.gmail.com]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_CAfile = /etc/postfix/cacert.pem
smtp_use_tls = yes
compatibility_level = 3.6
inet_protocols = ipv4
mynetworks = 127.0.0.0/8 [::1]/128 xxx.yyy.40.0/24
And made the proper sasl_passwd and cacert.pem files, everything works as expected.
Now I don't know why the new notification stack was unable to catch up the prune notifications.
Anyway... It's solved, for now.