While a warning dialog is built into Outlook 2013 and above, you need to use VBA or an add-in with older versions of Outlook to check for keywords which may indicate you planned to include an attachment.

For warnings before sending a message with a blank subject, see the following articles:
Outlook 2007 and earlier Outlook 2010 and above

Outlook 2013 and above

An attachment warning feature is built into Outlook, beginning with Outlook 2013. If you don't want the warning, you can disable it by checking the "Do not show... " box in the warning.

If you change your mind, you can turn it back on in File, Options, Mail. The option to warn before sending a message that may be missing an attachment is in the Send Messages section about halfway down.

VBA for Outlook 2010 and older

When you use Outlook 2010 and older, you need to use VBA or an add-in to warn about missing attachments.

The following macro needs to go in ThisOutlookSession and runs when the message is sent. If it finds the word "attach" in the message body, it triggers a warning dialog.

Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
   If InStr(1, Item.Body, "attach", vbTextCompare) > 0 Then
        If Item.Attachments.Count = 0 Then
          answer = MsgBox("There's no attachment, send anyway?", vbYesNo)
          If answer = vbNo Then Cancel = True
        End If
   End If
End Sub

Skip attachments in signatures

If you have an image in your signature, it will be detected as a an attachment and pass the Attachment check. To avoid that problem, check for attachment size. You'll need to loop through all attachments on the message and skip over the attachments that are smaller than a specific size. In my example, I'm looking for attachments over approximately 5 KB. If your signature has larger images, you'll need to increase the attachment size you check for.

Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
 If InStr(1, Item.Body, "attach", vbTextCompare) > 0 Then
    If Item.Attachments.Count = 0 Then
        answer = MsgBox("There's no attachment, send anyway?", vbYesNo)
    If answer = vbNo Then Cancel = True 
 End If

If Item.Attachments.Count > 0 Then
 For Each oAtt In Item.Attachments
    Debug.Print oAtt.Size
      If oAtt.Size < 5200 Then
        GoTo NextAtt
       
       Else
       answer = MsgBox("There's no attachment, send anyway?", vbYesNo)
       If answer = vbNo Then Cancel = True
    End If
NextAtt:
 Next oAtt
 
 End If
End If
End Sub

Check for attachment name in subject

We had an Outlook user sending wrong file (attachment) to wrong recipients. We created a template for each client that contains a unique Client number and the attachment filename always starts with the unique client number. By validating the subject line with the file name we can warn user the client number doesn't match so they can check before sending out the email.

This code checks the first 15 characters of the attachment filename against the first 15 characters of the subject.

By changing the If statement you could check other values.

Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
Dim objAttachments As Outlook.Attachments
Dim strAttachment As String

Set objAttachments = Item.Attachments

strAttachment = objAttachments.Item(1).FileName

 If Left(strAttachment, 15) = Left(Item.Subject, 15) Then
 Else
      prompt$ = "You sending  " & strAttachment & ". Are you sure you want to send it?"
       If MsgBox(prompt$, vbYesNo + vbQuestion + vbMsgBoxSetForeground, "Check Sending Account") = vbNo Then
         Cancel = True
       End If
 End If

End Sub

Combine Two ItemSend macros

If you want to use two ItemSend macros to check outgoing mail, you need to combine them into one. This example combines the "blank subject warning" and missing attachment macros.

We could do a better job of merging them into one macro, with one dialog box, but it will work simply by combing the two like this.

Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)

 Dim strSubject As String
  strSubject = Item.Subject
     If Len(Trim(strSubject)) = 0 Then
         Prompt$ = "Subject is Empty. Are you sure you want to send the Mail?"
       If MsgBox(Prompt$, vbYesNo + vbQuestion + vbMsgBoxSetForeground, "Check for Subject") = vbNo Then
        Cancel = True
      End If
    End If

   If InStr(1, Item.Body, "attach", vbTextCompare) > 0 Then
        If Item.Attachments.Count = 0 Then
          answer = MsgBox("There's no attachment, send anyway?", vbYesNo)
          If answer = vbNo Then Cancel = True
        End If
   End If

End Sub

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

To check your macro security in Outlook 2010 or 2013, 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.

After you test the macro and see that it works, you can either leave macro security set to low or sign the macro.

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