Overview

When a resource is tied up and cannot be used by another part of the mailserver process, the following error is shown in the Mail logs: "Result: delayed, Status: 4.3.0 Recipient's mailbox busy."

As a result, the user mailbox can't send or receive any emails. The email account in Kerio Connect Webmail returns the initializing icon and then times out. The Outlook for Mac configured via IMAP might experience intermittent connection fails, due to which there might also be a delay in incoming mails.

Solution

• These errors are a normal part of the mail server. If you receive temporary failures and issues, it is typically the result of some resource starvation.
• When the mailbox adds a new item to the folder, it has to update the index.fld file and several other files. If those files are large, the mail server exceeds the expected time to update them. When it tries to process the next item into the folder, the mail server finds that it is still busy from the previous message. Thus, it shows the mailbox busy error(s).
• When you notice this error, check if the folders with high reference counts have any additional users in the Users column of the Opened Folders.
  • In most cases, the username starts with the $ sign, as shown:
    

This error can be prevented by the various recommendations below:

• Resolving slow folder performance issues
• Use regular archiving
• The maximum suggested mailbox folder size is 20 GB. If you are over that limit, you can create additional folders to archive or sort the emails respecting the aforementioned limit. Note that the item count impacts performance more than the item size.
• Clean up the Deleted Items and the Trash folders regularly.

After these are done, follow the below steps to fix index.fld file issues. Administration access to Kerio Connect will be required for some of the steps:

1. User/s who are seeing the issue should log out of all the mail sessions, for example, Outlook, WebMail, etc.

2. Stop the Kerio Connect service (Windows, macOS/Linux).

3. Navigate to the Kerio Connect store folder.

4. Navigate to the <store>/mail/domain/user/<folder_name>/ directory.

5. Locate the index.fld file and rename it to index.bad.

   Note: The file extension must be renamed specifically to .bad.

6. Start the Kerio Connect service (Windows, macOS/Linux).

7. User/s who were seeing the issue should now login.

Testing

Monitor the performance and logs. The error is no longer shown in the Mail logs, and the performance of the Kerio Connect mailbox is back to normal.