Mu4e - Starting with version 0.9.8, mu provides an emacs-based e-mail client which uses mu as its back-end: mu4e.
grep - grep is a command-line utility for searching plain-text data sets for lines matching a regular...
Mutt - Mutt is a small but very powerful text-based mail client for Unix operating systems.
PowerGREP - Quickly search through large numbers of files on your PC or network using powerful text patterns to find exactly the information you want. Search and replace with plain text or regular expressions to maintain web sites, source code, reports, ...
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
dnGREP - dnGrep allows you to search across files with easy-to-read results.