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.

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 Initializing icon and then times out. The Outlook for Mac configured via IMAP might experience intermittent connection fails.

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Prerequisites

Admin access to the Kerio Connect server is required.

 

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Root Cause

These errors are a normal part of the mailserver. 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, along with several other files. If those files are large, the mailserver exceeds the expected time to update them. When it tries to process the next item into the folder, the mailserver finds that it is still busy from the previous message. Thus, it shows the mailbox busy error(s).

 

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Diagnosis

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 in the image below:
 

 

You can control this error by performing the following actions on the folders that have 10k items in them or more:

• Split all larger folders into smaller subfolders.
• Disable full-text search.
• Use regular archiving.
• Try to re-index the email account.

Note: It is advised to perform the actions that are listed above during the night/non-working hours to avoid any downtime.

 

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Solution

1. Log out of all the mail sessions, for example, Outlook, WebMail, etc.

2. Stop the Kerio Connect server.

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 server.

7. Log in again.

8. Monitor the performance and logs.

 

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Confirmation

The error is not shown in the Mail logs, and the performance of the mailbox is back to normal.

 

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Supplemental Information

Here is some useful information to prevent Recipient's mailbox busy issue:

• The default system folders, like Inbox and Sent Items, should not contain more than 10,000 items.
• The maximum suggested mailbox 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.
• The item count impacts performance more than the item size.
• It is helpful to clean up the Deleted Items and the Trash folders regularly.
• Extensive calendars can be caused by the duplication of the items within events, such as Attendee lines and MIME boundaries.

 

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