I read my mail through Gnus, an emacs-based news (Usenet, remember that?) and mail reader. (The main reason I do so is documented at Dan Barlow's diary, from an era when I routinely sent out patches by e-mail, and frequently sent out messages saying "please see the attached" while failing to attach anything to the message. Thmutt.)
In the intervening years, I've been accumulating mail accounts,
responsibilities, and I've often been frustrated by my inability to
search through non-local mail archives. It turns out that the reason
I've been unable to do so is largely my own fault: Kai Großjohann
nnir.el as an extension to Gnus, and Debian packagers
helpfully included that in the
packages. In the meantime, the Gnus developers quite rightly
nnir.el into their distribution, augmenting and extending
it over the years, and in particular giving it functionality to swap
out search backends depending on the mail server's capabilities.
Unfortunately, the 2003-era version of
nnir.el installed by
gnus-bonus-el was loaded in preference to that provided by Gnus
itself, in an otherwise fairly standard Debian installation of emacs.
It was only when I was looking at the source to see how much work it
would be to make multiple search backends possible (not too much but
more than an afternoon; it's lucky I don't charge by the hour) that I
noticed the date in the copyright header, and then that I had three
nnir.el installed... come to think of it, this is possibly
a sound argument for keeping the dates in the copyright headers of
source files up to date.
So, now, I can actually search (separately) my various personal and professional mail accounts. I don't think I can do terribly intelligent combined searches, and of course I still have to contend with Office365's dubious implementation of IMAP. But still, not a bad result.
Ironically, when I went to report all this upstream, I discovered that
just last week
gnus-bonus-el had been removed from Debian, with this
particular issue having been
reported three years ago.