The text entered exceeds the maximum length

In MSCRM 4.0, you may get the following error when trying to track an email:

The text entered exceeds the maximum length

This is caused because the message ID is too long for the field (though the error message is confusing, as it suggests the text entered is the issue when you’re not actually entering it). To resolve this:

Thanks to Oli Ward on the PSS at Microsoft for solving this.