I found some example code on the web, however the message "user defined type
not define appears on line "oOutlookApp As Outlook.Application"

Dim bStarted As Boolean
Dim oOutlookApp As Outlook.Application
Dim oItem As Outlook.MailItem

On Error Resume Next

If Len(ActiveDocument.Path) = 0 Then
   MsgBox "Document needs to be saved first"
   Exit Sub
End If

Set oOutlookApp = GetObject(, "Outlook.Application")
If Err <> 0 Then
   Set oOutlookApp = CreateObject("Outlook.Application")
   bStarted = True
End If

Set oItem = oOutlookApp.CreateItem(olMailItem)

With oItem
   .To = "recipient@mail.com"
   .Subject = "New subject"
   'Add the document as an attachment, you can use the .displayname
property
   'to set the description that's used in the message
   .Attachments.Add Source:=ActiveDocument.FullName, Type:=olByValue, _
     DisplayName:="Document as attachment"
   .Send
End With

If bStarted Then
   oOutlookApp.Quit
End If

Set oItem = Nothing
Set oOutlookApp = Nothing