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.
Mu4e - Starting with version 0.9.8, mu provides an emacs-based e-mail client which uses mu as its back-end: mu4e.
Noizio - Feeling that the noise of the big city is too much for you to bear?
Mutt - Mutt is a small but very powerful text-based mail client for Unix operating systems.
MyNoise - Custom shaped online noise machines. Many Many generic sounds
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.