Symptom is no email is being received
tail /var/mail/root contains the following error:
Diagnostic-Code: X-Postfix; unknown user:
When you install postfix if your hostname includes your domain then Postfix thinks its receiving mail for the domain, to fix:
vim /etc/postfix/main.cf
remove [yourdomain.com] from mydestination section
restart postfix and retest
postfix stop
postfix start
Reference:
http://stackoverflow.com/questions/32877196/postfix-thinks-the-system-email-is-local