Late, but yet another Fastmail user, with Gnus.
Like the other comment says, the feature you are looking at is Aliases, I have my primary address and then subscriptions@mydomain.com, work@mydomain.com, etc with rules that send them to different directories.

I wrote a package to sync Fastmail’s CalDAV calendars to Emacs’ built in calendar, and now I can user FM good apps on the phone and all their services in Emacs when on the computer.

Will check out my system for more info manuals (and distro repos, I use Fedora).

At $NEWJOB I am back to using Windows, thankfully with Emacs I can still access the manuals easily.

Worded that poorly, I am looking for things in addition to the Emacs, Elisp, etc manuals.
Non GNU project manuals, if you will.

Wow TIL about the occur behavior with prefix arg.

I usually call my own wrapper command that runs occur for the selected region (instead of limiting to region) or the symbol at point (the most common case when programming). Thanks to your video I added prefix arg support:

(defun hoagie-occur-sexp-or-region ()
  "Run occur for the sexp at point, or the active region.
By default, occur _limits the search to the region_ if it is active."
  (interactive)
  (occur (if (use-region-p)
             (buffer-substring-no-properties (region-beginning)
                                             (region-end))
           (thing-at-point 'sexp t))
         (when current-prefix-arg
	       (prefix-numeric-value current-prefix-arg))))

Thank you!!!