Emails sent to a Plesk server are deferred: Unable to read message content

Symptoms

All inbound emails are deferred by Postfix. The following messages appear in /var/log/maillog.

status=deferred (Unable to read message content )
...
postfix-local: cannot create temporary file - (30) Read-only file system

Cause

Postfix is installed from OS vendor repository:

# rpm -qi postfix | grep Packager
Packager : CentOS Buildsys bugs@centos.org

Resolution

1. Connect to the Plesk server via SSH.

2. Open the file /usr/lib/systemd/system/postfix.service in a text editor. In this example, we are using the vi editor:

   # vi /usr/lib/systemd/system/postfix.service

3. Comment the following lines out by placing a # (hash) character at the beginning of the line:

   #PrivateTmp=true
   #CapabilityBoundingSet=~ CAP_NET_ADMIN CAP_SYS_ADMIN CAP_SYS_BOOT CAP_SYS_MODULE
   #ProtectSystem=true
   #PrivateDevices=true

4. Save the changes and close the file.

5. Reload systemd unit file:

   # systemctl daemon-reload

6. Restart the Postfix service:

   # systemctl restart postfix