Mail client provides warnings regarding IMAP connection for Plesk mailboxes: inotify initialization error

Symptoms

• Emails from a Plesk server can not be sent using a mail client, for example, Outlook:

  Your IMAP server wants to alert you to the following: Inotify initialization error

• Courier IMAP server is used for IMAP/POP3 delivery.

• In the /var/log/maillog log file the following errors can be found:

  plesk courier-imaps: Failed to create cache file: maildirwatch (jdoe@example.com)
  plesk courier-imaps: Error: Too many open files

Cause

Insufficient value for per-userid limit of inotify file descriptors.

Resolution

1. Connect to the server via SSH.

2. Check the current per-userid limit of inotify file descriptors:

   # cat /proc/sys/fs/inotify/max_user_instances
   128

3. Increase this value to a greater one via the command below, for example to 200:

   # echo "fs.inotify.max_user_instances=200" >> /etc/sysctl.conf

   Note: Rough metric is the number of concurrent IMAP sessions multiplied by 4.

4. Apply changes by executing the command:

   # sysctl -p

5. Restart Courier-Imap services:

   # systemctl restart courier*