emails – Why is Drupal not sending mail?

When submitting a contact form there is no mail sent, eventhough there is a confirmation message. Or when trying to send a password reset link, everything goes well, Drupal says “If MYMAIL@MYDOMAIN.XX is a valid account, an email will be sent with instructions to reset your password.” but no mail is sent.

  • Sendmail is installed, tested with PHP mail() and working fine.

  • Tested Sendmail through the command line and working fine: echo "Subject: sendmail test" | sendmail -v MYMAIL@MYDOMAIN.XX

  • Testing via drush also works fine with: drush php-eval "print mail('MYMAIL@MYDOMAIN.XX','Test subject from drush','Test message','From: drush@MYSERVER');"

  • Installed helper module that outputs in watchdog ‘sent’ message, but no mail is sent

  • When submitting forms in Drupal there are no entries in mail.log or syslog

Edit 1: this is a live server. There is no Mail Safety module installed. I would like to use the existing Sendmail as it is used succesfully on my other servers with Drupal sites.

Edit 2: Some other Drupal sites on the same server are succesfully sending mail, no idea why and why this one is not…

Edit 3: The Drupal sites that are not sending mail are test sites TEST.DOMAIN.XX
The sites that DO send mail are the normal ones, WWW.DOMAIN.XX

Why is Drupal not sending mail and how could I debug this problem further? Thanks