Just for the records. The mail issue is fixed.
As @ClaudioSpcSpa wasn’t using neither clamav, spamassassin nor sieve, the fix has been easy:
cp /usr/local/hestia/install/deb/exim/exim4.conf.4.95.template /etc/exim4/exim4.conf.template
echo "$(head /dev/urandom | tr -dc A-Za-z0-9 | head -c16)" > /etc/exim4/srs.conf
chmod 640 /etc/exim4/srs.conf
chown root:Debian-exim /etc/exim4/srs.conf
systemctl restart exim4