Information is definitely out-dated, and you shouldn't really take it to heart.
This feature is new in R2DA, and was not in past Reason 2 Die games. This added a side objective to do in-game, though they didn't serve an important purpose.
When you received an achievement, it would appear in your mail, the only known use of the mail system.
Join your first game of Reason 2 Die Awakening and get a free gift.
Reward: 120$ Cash
[Broken Before Removal] : Get a friend to follow you in R2DA for the first time and get some free gifts.
Rewards: 100$ Cash, 100 Experience, and 2 Fame
Broken Reason: Universes; cannot follow a friend into R2DA for the first time without going through the choice universe.
[Scrapped] : Beat your old record on any Survival or Hard map to get a free gift!
Reward: 100$ Cash
Reason Removed: It is actually unknown why this was removed, but it is estimated that it was removed because you could easily reset every 1% to earn 100$ Cash, which would earn 10,000$ Cash per map and/or Gamemode.
Welcome to Reason 2 Die Awakening
[DEMO Only] : Claim your beginners REWARD by right clicking the letter icon and choosing delete.
R2DA Opening Event
[DEMO Only] : This month we are happy to release R2DA out of demo. We celebrate this by giving out generous [sic]
- The Friendly Recruiter achievement could formally be easily exploited through the use of alternative accounts, and a couple of computers.
- Removed in v0.9.6.
- This is technically in Debug, as in some servers, the mail system hasn't been removed yet.