Visual Studio 2010, Outlook 2010 And Windows Application, Return Emails?

At present I have wrote an application (in-house CRM, vb .net 2010) which allows me to send emails under the selected customer & I categorised these emails.As these are categorised I can return data to display email history in my program by using search criteria:

Dim oMail As Outlook.MAPIFolder = oNS.GetDefaultFolder(Outlook.OlDefaultFolders.olFolderSentMail)
Dim sSearch As String
sSearch = "[Categories] = '" + "[" + tAccount.Text.Trim + "]" + "(" + tShipTo.Text.Trim + ")'"
Dim oItems As Outlook.Items = oMail.Items.Restrict(sSearch)

However the limitation to the above is it only looks at sent items & no other folder.My ideal solution would be to display all email correspondence for a certain contact.