Just checked /var/log/mail.log and it contains repeated messages like this
Mar 20 08:30:01 ip-x-x-x-x sSMTP[2408]: Creating SSL connection to host
Mar 20 08:30:01 ip-x-x-x-x sSMTP[2408]: SSL connection using RSA_AES_128_CBC_SHA1
Mar 20 08:30:02 ip-x-x-x-x sSMTP[2408]: 554 Transaction failed: Missing final '@domain'
Edit: Seems to be related with Cron job don't send emals with Amazon SES