Fixing Large Kerio Mailboxes

I bet you have customers who never delete anything out of their Inbox, or if they do, they never empty their deleted items folder. If they are using Outlook PST files, they will sooner or later reach the maximum file size possible and their mail will break (actually, with Outlook it will usually break before that). Sometimes you can fix it with SCANPST.EXE, sometimes you can't.

If you are using Imap or Kerio Outlook connector, you won't see quite the same problem. Kerio stores mail messages individually on the server, so you are never in danger of reaching file size limits - well, technically you still could because the directory itself could grow too large, but that would take an unimaginable number of messages.

Even so, having a lot of messages is a problem. It slows things down: if you have 40,000 messages in your INBOX, think about what happens at the system level when a new message arrives. That directory with 40K items already in it has to be scanned to find a free spot to stick the new entry. It takes time to scan a big directory; if you have dozens of users all with giant INBOX files, the arrival of new email can really slow the system down. That's just the OS: Kerio itself maintains indexes of the messages, and every new message requires adding to that ever growing index.

Note that there's nothing wrong with having large mail folders. It's having large mail folders that need to be accessed frequently (INBOX, Junk Mail, Deleted Items) that is the problem.

So what's the solution? Well, you could just drag and drop a bunch of messages into a new folder, and if you had been doing that regularly before you got up to tens of thousands of messages, that's fine. But once you get a very large directory, it stays large even when it becomes mostly empty - it still takes extra time to scan through it. So what to do?

With Kerio, the solution is pretty simple. Stop the mailserver. That's usually done with the Kerio Engine Monitor, though on Linux you can do it with "/etc/init.d/keriomailserver stop". Now rename the oversized directories to something else. You might rename INBOX as "Inbox200805", for example. Do the same for large Delete Items and Junk mail folders. Restart the server; new mail will cause Kerio to automatically create any folders it needs (INBOX, etc.), but they will now be small and quickly scanned.

It's that easy, and it can really improve performance.

As with anything like this, be sure to have adequate backups before implementing this procedure.