This Visual Basic for Applications example uses the Execute method to look through all the actions for the given mail message, and executes the action called "Reply to All."
Set myOlApp = CreateObject("Outlook.Application")
Set myNameSpace = myOlApp.GetNameSpace("MAPI")
On Error GoTo ErrorHandler
Set myItem = myOlApp.ActiveInspector.CurrentItem
For Each a In myItem.Actions
If a.Name = "Reply to All" Then
Set myItem2 = a.Execute
Exit For
End If
Next a
Exit Sub
ErrorHandler:
MsgBox "There is no current item!"