Run macro when email is sent - vba

Question

I am trying to write a macro that looks at the subject line of an email whenever the user hits the Send button.
However I can't find any documentation that listens to that button. For right now I am just trying to get it to send a MsgBox with the subject when the email is sent. Is there a way to listen (thinking in terms of DOMs) to this button and fire a macro on the click.

Answer 1

As suggested by Siddharth:
I have written a small demo which checks some conditions to decide, if the send operation should be canceled. This could be extended to do other things like inserting dates, saving the mails to some folder, ...
Option Explicit
Private Sub Application_ItemSend(ByVal objItem As Object, Cancel As Boolean)
Dim mi As MailItem
If TypeName(objItem) = "MailItem" Then
Set mi = objItem
Debug.Print mi.Subject
check Cancel, Trim(mi.Subject) <> "", "Subject is empty!"
check Cancel, Not isRecipient(mi, "John#Doe.net"), _
"John is on our embargo list!"
End If
End Sub
Private Sub check(ByRef Cancel As Boolean, cond As Boolean, msg As String)
If Not (Cancel Or cond) Then
Cancel = (MsgBox(msg & vbCrLf & "Cancel send operation?", _
vbYesNoCancel, "Confirm?") <> vbNo)
End If
End Sub
Private Function isRecipient(mi As MailItem, forbidden As String) As Boolean
Dim ret As Boolean
Dim rc As Recipient
ret = False
For Each rc In mi.recipients
If StrComp(rc.Address, forbidden, vbTextCompare) = 0 Then
ret = True
Exit For
End If
Next
isRecipient = ret
End Function