www-data@hostnaname.example.com F=<>: Unrouteable address

Test routing path:

exim -bt www-data@example.com

DEBUG: system_aliases for www@example.com
DEBUG: procmail for www-data@example.com
www-data@example.com is undeliverable: Unrouteable address

So the reason of problem is suffix based aliases search pattern. Address www-data is translated to www which doesn't exists.

Workaround: add some workaround aliases like:

www@example.com: user
www: user:

...
2019-07-13 09:14:44 unable to set gid=33 or uid=0 (euid=0): forcing real = effective
2019-07-13 15:38:11 unable to set gid=33 or uid=0 (euid=0): forcing real = effective
...

Reproduction:

mailtest.php

<?php
    $email = "user@domain.com";
    $subject = "test z phpa";
    $message = "lorem ipsum";
 
    $mail = \mail($email, $subject, $message);
    print ($mail);
?>

Reason:

Apache Module MPM ITK The MPM ITK Apache module implements restrictions on the use of the setuid() function and the setgid() function. As a result, scripts that depend on these functions may encounter problems. This includes scripts that use the mail() function, the shell_exec function, or the sudo command.

You can resolve these restrictions with one of the following methods:

• Do not use the MPM ITK Apache module.
• Update your script to no longer require escalated privileges.
• Turn off the security and allow users to execute scripts as the root user. You can allow users with a UID or GID between 0 and 4294496296 to bypass security if you add the following code to your /etc/apache2/conf.d/includes/pre_virtualhost_global.conf file.

    <IfModule mpm_itk.c>
    LimitUIDRange 0 4294496296
    LimitGIDRange 0 4294496296
    </IfModule>
    

  
  **Warning:**  We strongly recommend that you do not enable root privileges for your users. This action has major security implications and could endanger your server.

To work around it, you would have to change your php or php-apps to send mails via SMTP to localhost instead of invoking /usr/lib/sendmail directly. I can't give any advice on how to do that, though. Or maybe there is some way to install some minimal MTA like ssmtp or nullmailer as /usr/lib/sendmail that forwards the mail to exim via SMPT. Not sure how easy it is to install those parallel to exim, though. Maybe it could work by diverting exim's /usr/lib/sendmail .