Given that you have posted in an Outlook forum I assume we can expect that the macro is to be run in Outlook VBA, in which case you don't have to create a new Outlook application, you are already in Outlook.
You don't need to select a message in order to process it, you merely have to identify it to the macro. The following will identify the latest item in the sent mail folder.
Your Userform is a mystery so I cannot comment on that.
Private Sub SentSave()
Dim olItems As Items
Dim olItem As Object
Set olItems = Session.GetDefaultFolder(olFolderSentMail).Items
olItems.Sort "[Received]", True
'<here I need to have the last item sent highlighted in the sentmail folder
Set olItem = olItems(1) 'You have it
'save emails to job folders using listbox
UserForm1.Show
End Sub