I've used grep to search for strings across all my mailboxes for decades. Found out today, it randomly doesn't work based on whether or not the file contains any text that doesn't comply with POSIX. So if one user has UTF-8 encoding and another, ISO-8859-1, and they are in the same mailbox, according to POSIX, that's a binary file. You have to tell grep to search (and potentially display) binary data -- which can easily through a terminal into weird modes, making it unreadable (see attached example for results of a random binary being listed). Note the last line is the prompt same text as you can see at top of window. mbox files don't do this when you search for strings because when I search for strings I'm looking for something in the text of an email. While I want grep to skip things like compressed files and coredumps, I don't want it judging the quality of "text" that I'm searching through -- but that's what many of the utilities have been modified to do -- if it doesn't fit the POSIX definition of text, then some text utils won't process it. Technically, if the last line of the file doesn't end with a newline, it's also binary (though grep still displays it). Many text utils used to be generally useful -- but now they are having functionality removed to have them only work with POSIX. I suppose no one else really does a quick search through all their email this way any more. Though is this what you'd expect? Sigh.
