Sorry, I think it colud be a problem on ssmtp.conf template or with the cron jobs.
I solved the problem with two solutions found googling:
The problem is SES requires a domain in both the "from" and "to" fields
From here solve the TO address: adding a
MAILTO="firstname.lastname@example.org" in cronfle
#Ansible: client WordPress cron
*/15 * * * * web cd /srv/www/client/current && wp cron event run --due-now
From here solve FROM addres:
When ssmtp is installed, it creates a link from /usr/sbin/sendmail to /usr/sbin/ssmtp. What I did was delete the link and created a sendmail file that contains the following:
sed "s/From: root /From: email@example.com /" | /usr/sbin/ssmtp $*
Now any mail sent from root will get rewritten to firstname.lastname@example.org
Finally, if you don't want receive emails from Cron Daemon at every execution you can set
MAILTO="" in the cronfile.