One of the form options on the New menu is "New Post in the folder", an option which is highly underused. Posts are non-mailable messages which are saved to the currently selected mail folder. Use posts for notes to yourself or to hold snippets of information. (Posts often a better choice for longer notes than Outlook's Notes).
You can easily edit a post by opening it and choosing Edit, Revise contents, or Reply and create a second post. Search for posts by sorting the icon column in the message list or use Advanced find's Advanced tab to search for Message class of IPM.Post.
At one time, Microsoft released a sample diary application that used a customized post form so you could use Outlook as a diary. While a custom form isn't necessary to keep a diary, it was cute and provides a basis for your own customizations.
While the form itself works with all versions, the automatic installation may not. Open the diary.pst using File, Open and publish the form to the folder you want to use it with. If you'd like to try the diary form and can't find it at Office Downloads, a copy is available at https://www.outlook-tips.net/files/diary.exe .
Published March 16, 2004. Last updated on October 31, 2011.