grepWin - grepWin is a simple search and replace tool which can use PCRE regular expressions to search for...
Mu4e - Starting with version 0.9.8, mu provides an emacs-based e-mail client which uses mu as its back-end: mu4e.
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, ...
Mutt - Mutt is a small but very powerful text-based mail client for Unix operating systems.
dnGREP - dnGrep allows you to search across files with easy-to-read results.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.