This is v2.0 of my mailbox delivery notification system. In this version I scrap the XBee radios, and notification light for an ESP8266 and email notification of delivery. With exception of switching out the Arduino Pro Mini for an ESP8266 and an ATtiny85, all the other core components and construction are the same as v1.0.
Decent tutorial on setting up Postfix as a relay – I don’t recommend making your server publicly accessible unless you know what you’re doing. If your project does not require secure email, use a service like SMTP2go.
DPST relay (use/need is described in video).