CMSs not sending emails (but php mail works)

[sylvaticus]

Hello I have a Ubuntu 14.04 VPS with a few sites.
If I run

php -r "mail('mytest@emailaddress.org, 'test', 'test');"

this goes delivered, but I have a couple of CMS (Drupal and dokuwiki) that doesn't send any emails, including basic password reminder/registration.
I have checked and I am not blacklisted.
Where should I look to start debugging ?

Edit: it seems the problem is that I have the incoming emails using a third party while postfix try to deliver the email locally (maybe looking at the alias file?).
How can I force postfix to NOT try to deliver the email locally ?

[sylvaticus]

The solution is to remove $mydomain (or "myownservername.xxx" if set explicitly) from mydestination variable in /etc/postfix/main.cf.

See also: http://serverfault.com/questions/249...39d4194bea7fd7