Some custom or third-party solutions change the default message class that is used when new items are created in a Microsoft Outlook folder. The message class is associated with each item in a folder. Additionally, it defines which form to display when the item is opened. If a custom solution changes the default message class for an Outlook folder, uninstalling that solution may not reset the folder's new-item message class to the default configuration. In this situation, various issues can occur in Outlook, because the default new-item message class for a folder was changed from the default value. For example, if you click the New button to open a new item for the folder, the form does not seem to be the original default form that was installed by Outlook.
If you are not using a custom Outlook solution that requires a custom message class for new items in a folder, we recommend that you use the default message class for at least the following primary Outlook folders:
The following table lists the main default folders in Outlook in which custom messages classes are sometimes found. The second column in the table provides the default message class that is used for the folder.
Default message class
To determine the currently configured default message class for new items on a folder, follow these steps:
Right-click the folder, and then click Properties.
Select the General tab.
Inspect the value that appears in the When posting to this folder, use list.
If you want to change the default message class for new items that you add to the folder, click the When posting to this folder, use list, and then select the appropriate message class for the folder. (See the table.)
Note If you already created items in the folder by using a custom message class, changing the When posting to this folder, use value does not change the message class on these existing items. When you change the When posting to this folder, use value, only items that are created after the change will use the newly configured message class.
For more information about how to change the message class on existing items in a folder, click the following article number to view the article in the Microsoft Knowledge Base:
201087 How to update existing items in an Outlook folder to use a new custom form