Você pode executar um script.
O VBA seria algo como isto.
Sub MultipleDelayedReplies(MyMail As MailItem)
Dim HourInc As Integer
Dim sEndDate As Date
Dim reply As MailItem
Dim i As Integer
sEndDate = Now()
reply.Subject = reply.Subject & " Immediate Reply."
reply.HTMLBody = "Reply 1" & reply.HTMLBody
HourInc = 3
For i = 2 To 5
Set reply = MyMail.reply
sEndDate = DateAdd("h", HourInc, sEndDate)
reply.DeferredDeliveryTime = sEndDate
reply.Subject = reply.Subject & " " & reply.DeferredDeliveryTime
reply.HTMLBody = "Reply " & i & reply.HTMLBody
'reply.Display
' or
reply.Send
Next i
End Sub
O processo é mais descrito aqui.
Acredito que o Outlook precisa estar ligado para enviar os e-mails nos horários designados.