crontab error - SSMTP - 553 5.7.1 Sender address rejected: not owned by user

As root, I installed ssmtp and I configured the /etc/ssmtp/ssmtp.conf as follows:

# Sender email address
root=myemail@domain.com

# Destination SMTP server and port
mailhub=mail.domain.com:587

# Username and password
AuthUser=myemail@domain.com
AuthPass=password

# Sender domain
rewriteDomain=domain.com

# Machine's hostname
hostname=mail.domain.com:587

# Allow set From name in each email
FromLineOverride=YES

UseSTARTTLS=YES
UseTLS=YES

I also configured revaliases in /etc/ssmtp/revaliases adding the following row: root:myemail@domain.com:mail.domain.com:587 I set a cron running crontab -e and added the rows (just to test it's running):

MAILTO=myemail@domain.com
* * * * * echo "this is a test"

If I run grep cron /var/log/syslog I see the following error:

cron: sendmail: RCPT TO: (553 5.7.1 : Sender address rejected: not owned by user myemail@domain.com)

myemail@domain.com is changed in root@domain.comand I cannot find a solution. Any help?