I was trying to set up `aerc` to use `lynx` instead of `w3m` as the html
filter and managed to do this with `text/html=lynx -dump -stdin`.

However I noticed the original filter also wraps the commands with
`socksify`. I am not knowledgeable when it comes to the SOCKS protocol
so could someone be kind enough to explain what this actually does? The
accompanying comment says it "prevents the emails from phoning home"
which suggests some sort of firewalling and the man page says "socksify
is meant to aid in using socks with ... binaries" which isn't
particularly illuminating if you don't know what "using socks" this way
entails.

Apologies for asking a question not directly related to the aerc code
but I'd like to understand why exactly it chooses to call this program.
Thank you for your patience and help.

SOCKS is a proxy protocol, and socksify redirects all internet traffic from a program to the specified proxy server (passed as an env var, for example).
In this case, the HTML filter is redirecting all potential requests to localhost, which most likely will fail, thus preventing any phoning home from e-mails.
So, to make your lynx filter, you should keep the socksify wrapper and only change the w3m call.

I hope this helps! :)