As there was no email coming from a VM, I checked /var/log/maillog:

Jun 25 10:59:13 localhost postfix/local[20793]: fatal: open database /etc/aliases.db: No such file or directory

As the missing file is some form of (Berkley?) DB, a simple touch doesn't create any structure for a suitable "empty database".

To fix, update the /etc/aliases database with newaliases:


From the man page:

This is a link to the ssmtp binary. It invokes /usr/sbin/sendmail with the -bi option. It is provided for compatibility with the sendmail program. In this case it does absolutely nothing since sSMTP does not support /etc/aliases and is just there to avoid programs returning error messages.