In a long thread discussing the problem of Outlook not showing the recipients email address at How to show email address not just name in From and To fields a user mentioned the steps he uses to get around this problem:

I need the full email address displayed when I print a sent email so that I can prove exactly what email address it was sent to. So instead of TO: 'John Doe' it will print out as john@domain.com. Right now when replying to an email, or starting a new one, I have to double click on 'John Doe ' and copy the full email address and paste it into the TO field. Otherwise it will print out as John Doe.

Outlook uses the display name in the To/CC/BCC fields, getting it from the message you are replying to, or from the Email Display Name field in Contacts. Newer versions of Outlook will use Full Name (email@domain.com) as the display name format in Contacts but senders almost always use their name as the display name and don't include the email address. While newer versions of Outlook display the email address when you are composing mail, when the message is sent, only the display name is visible.

This specific problem is easy to solve using an ItemSend macro. Before the message is sent, the macro changes display names to the underlying email address. If the recipient entry contains an @ sign, it's skipped (remove the If/End If lines if you use @ signs in display names). It's completely automatic - as soon as you press Send, the display names are changed to the email address.

December 4 2018: Updated macro to check for To, CC, and BCC and put the addresses in the correct field.

Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
Dim Recipients As Outlook.Recipients
Dim R As Outlook.Recipient
Dim i, ToCC, newR

Set Recipients = Item.Recipients
For i = Recipients.count To 1 Step -1

Set R = Recipients.Item(i)
' if the entry is already an address, skip it
If InStr(1, R, "@") = 0 Then
ToCC = R.Type
Debug.Print R, ToCC
   Set newR = Recipients.Add(R.Address)
  newR.Type = ToCC
   Recipients.Remove i

End If

Next
Recipients.ResolveAll

End Sub

How to use the macro

First: You will need macro security set to low during testing.

To check your macro security in Outlook 2010 or above, go to File, Options, Trust Center and open Trust Center Settings, and change the Macro Settings. In Outlook 2007 and older, it’s at Tools, Macro Security. If Outlook tells you it needs to be restarted, close and reopen Outlook. Note: after you test the macro and see that it works, you can either leave macro security set to low or sign the macro.

Now open the VBA Editor by pressing Alt+F11 on your keyboard.

To use the macro code in ThisOutlookSession:

1. Expand Project1 and double click on ThisOutlookSession.
2. Copy then paste the macro into ThisOutlookSession. (Click within the code, Select All using Ctrl+A, Ctrl+C to copy, Ctrl+V to paste.)

More information as well as screenshots are at How to use the VBA Editor.