No it is not a treeview it is a cross between a treeview and a listview:)
Yes I have been using the listview groups since 2.0 and they are very nice. The +/- icon to expand/collapse is to the left of the group header (and it even looks like M$ made room for the +/- icon there).
The more I look at the Outlook interface it seems like they are using a grid control and not a listview control because of the numerous icon columns and such.

Here is an example of that I am looking for (and it is not available natively with listview control)

- Group header Text 1
Listview item 1.1
Listview item 1.2
Listview item 1.3
Listview item 1.4
- Group header Text 2
Listview item 2.1
Listview item 2.2
Listview item 2.3
Listview item 2.4

If you click the '-' icons to the left of the "Group header text" it would look something like this

+ Group header Text 1
+ Group header Text 2

>Einar,
>
>I don't use Outlook, so I can't actually look at an example of what you're talking about, but it sounds more like TreeView behavior than ListView. But, I just found, via Intellisense, something called ListViewGroups. So I went into the docs and found that it is something new to .NET 2.0 ... first I've heard of it, but I bet this is what you're looking for. If I have any free time this weekend, I might play with it and see.
>
>~~Bonnie
>
>
>
>>I do not think this feature is available right out of the .NET box, but feel free to correct me if I am wrong.
>>
>>In the detail view of Outlook there is a way to group my messages by the day they were received and I can expand/collapse that group by clicking on a +/- ro the left of the group title.
>>
>>Has anyone seen an implementation of this along with some source code perhaps?
>>
>>(I'll try to google for it as soon as I figure out the keywords to search for :)