The One-Click-Mail (abbreviation: OCM) extension allows a player to collect all packages of mail at the player's Main Station, or at a neighbour's Main Station, by clicking only one of the packages. This eliminates having to find and click each individual package.
Note that One-Click-Mail does not affect the ability to click on mail wagons to unload them. To unload mail wagons with one click, purchase the One-Click-Unload (abbreviation: OCU) extension. OCU allows unloading of all trains with one-click.
Added as part of the 14 Jul 2011 Game Update.
At one point, Pixel Federation offered One-Click-Cargo, One-Click-Passenger, and One-Click-Mail for free to new players who played the game via the TrainStation Portal. However, they realized that this made the game unbalanced, and they removed this incentive. Pixel still promote the Portal as the preferred method of playing the game, using Portal-only content as incentives.