Can only send email via Outlook if Outlook is open

I want to use send emails via Outlook as described here. It works fine as long as I have already opened Outlook. So for example if Outlook is minimized and I execute my code, then I can send an email just fine. But if Outlook is closed, then I get an exception:

{System.Runtime.InteropServices.COMException (0x80004004): Operation aborted (Exception from HRESULT: 0x80004004 (E_ABORT))
   at Microsoft.Office.Interop.Outlook._MailItem.get_Recipients()
   at OutlookExample.Form1.btnSendEmail_Click(Object sender, EventArgs e) in C:\Users\abc\Documents\Visual Studio 2008\Projects\OutlookExample\OutlookExample\Form1.cs:line 28}

Here is the code:

using Outlook = Microsoft.Office.Interop.Outlook;

...

private void btnSendEmail_Click(object sender, EventArgs e)
{
    try
    {
        Outlook.Application oApp = new Outlook.Application();
        Outlook.MailItem oMsg = Outlook.MailItem)oApp.CreateItem(Outlook.OlItemType.olMailItem);
            oMsg.HTMLBody = "Hello, here is your message!";
            oMsg.Subject = "This is a test message";
            Outlook.Recipients oRecips = (Outlook.Recipients)oMsg.Recipients;
            Outlook.Recipient oRecip = (Outlook.Recipient)oRecips.Add("rhr@sonlinc.dk");
            oRecip.Resolve();
            oMsg.Send();
            oRecip = null;
            oRecips = null;
            oMsg = null;
            oApp = null;
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.ToString());
    }
}

Why doesn't this work?

Edit: Here is the solution

using Outlook = Microsoft.Office.Interop.Outlook;

...

private void btnSendEmail_Click(object sender, EventArgs e)
{
    try
    {
        Outlook.Application oApp = new Outlook.Application();

        // These 3 lines solved the problem
        Outlook.NameSpace ns = oApp.GetNamespace("MAPI");
        Outlook.MAPIFolder f = ns.GetDefaultFolder(Outlook.OlDefaultFolders.olFolderInbox);
        System.Threading.Thread.Sleep(5000); // test

        Outlook.MailItem oMsg = Outlook.MailItem)oApp.CreateItem(Outlook.OlItemType.olMailItem);
            oMsg.HTMLBody = "Hello, here is your message!";
            oMsg.Subject = "This is a test message";
            Outlook.Recipients oRecips = (Outlook.Recipients)oMsg.Recipients;
            Outlook.Recipient oRecip = (Outlook.Recipient)oRecips.Add("rhr@sonlinc.dk");
            oRecip.Resolve();
            oMsg.Send();
            oRecip = null;
            oRecips = null;
            oMsg = null;
            oApp = null;
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.ToString());
    }
}

The following code has reliably worked for months for me:

            app = new Microsoft.Office.Interop.Outlook.Application();
            Microsoft.Office.Interop.Outlook.NameSpace ns = app.GetNamespace("MAPI");
            f = ns.GetDefaultFolder(OlDefaultFolders.olFolderInbox);
            Thread.Sleep(5000); // a bit of startup grace time.

if outlook was open it uses it, if not its opened it. Of course, if your outlook requires you to login, your code wont allow for that. Some systems make it difficult for you to auto login.