Noisli - Noisli is a fantastic background noise and color generator for working and relaxing. Online soothing ambient sounds like White noise, Rain and Coffee Shop.
Geary - Geary is an email application built for GNOME 3.
Noizio - Feeling that the noise of the big city is too much for you to bear?
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Rainy Mood - Rainy Mood is the world's most popular rain simulator. Sleep, study, and relax with the sound of rain.
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.