@zgokan Can you show a screenshot of composing the email in zimbra.
Also, can you confirm you have configured the zimbra backend to load - it will not auto-load like other Kopano supplied backends
in config.php (usually linked to /etc/z-push/z-push.conf.php you should have
// the backend data provider
define('BACKEND_PROVIDER', 'BackendZimbra');
How large is the image?
Have you checked your z-push.log file and your web server log files to see if there are errors?
You could generate a log file for the affected user showing a full cycle, starting with a Ping, send a new message that will cause the issue, you will see the Sync to export the message to the device, followed by another Ping.
Make sure you set the loglevel to at least debug for the special log file, and you add the same userid as is configured on the device - either a full email address or a username
define('LOGUSERLEVEL', LOGLEVEL_DEBUG);
$specialLogUsers = array();
$specialLogUsers[] = 'user@domain';
OR
$specialLogUsers[] = 'user';
This should create an extra log file called user_domain.log or user.log
Please share that log file