I thought I would share my solution for how I wired my mailbox into smartthings. I was at a disadvantage in that my mailbox is 140 feet (direct line of sight) from my hub, this includes going through numerous walls and my neighbors house as our mailbox is grouped with a few others in front of their house (it is between my neighbors house and their neighbor). In addition, they are steel mailboxes on a wood post. Range was the number one issue outside of obvious concerns such as weather.
Parts list:
GE 12721 (needed for my setup to extend my mesh network)
Ecolink door contact sensor
Ademco 945T-WH magnetic contacts
3 feet of doorbell wire
To solve my distance issues, I had to wire a GE 12721 outlet to my neighbors front door, this was just close enough act as a repeater. So Step 1 to resolve a distance issue, have an awesome neighbor. To ensure that the steel mailbox didn’t block the ecolink sensors signal, I mounted a weatherproof gang box to the back of the mailbox post, drilled a whole through the back of the box and the post so I could feed the wire through to hide everything. The ecolink sensor is mounted in the box, wire fed through and underneath my mailbox. I drilled a small hole by the door to feed the wire inside, and then mounted the ademco contacts.
Had some issues at first and discovered that my neighbors outlet I added was just far enough to have intermittent connectivity issues, so I had to put another repeater on the corner of my house (not a huge problem). But now everything works perfect. I have the “Notify me when” setup to send me my “Mail has Arrived!” notification whenever the door opens 
Thought I would share my approach to this as I have distance issues + steel mailbox issues.
