Various said:
signing
Is never a problem, so long as it's done according to spec so MUA's can handle it correctly.
signing means badness
Signing does not dictate or modify the signed content. If your signing tool does, it's broken, go find a new one. If you author shit eqiquette to sign, go learn etiquette.
short and long links both present
Redundant, post the long canonical one instead. Wrapping long links is bad.
wrapping quoted
You didn't make the error, so you don't have to correct it, and doing so may introduce additional errors / info loss. Any 'auto wrappping of quoted' function of MUA's / editors is best left turned off given all the garbage input they have to deal with.
html vs text
HTML is bloated bling, if it can't be said in text it's probably not worth saying. If you want a weblog, print rag or photo art design studio... go make one, and link to it. It if was good, people might request subscribe to your email version.
obfuscation
This does nothing adversaries can't and haven't figured out. If you want private, go e2e crypto and pray you're not ratted.
fonts
Support for UTF-8 is a bit different than that. Downconverting to ASCII replaces information with garbage. Don't do that. Upgrade your OS, tools, and configs instead.
image / data attachment sizes
Shrink them back to Nixon era TV quality. An under 20k image tells more or less the same story as a 1M one. If it's science detail or whatever else you want to show with it... crop out the zoom, or link to it, or ask. Same for data. Of course there are exceptions for first emergency actions, yet those are rare events. Anything else, done with regularity, is offloading onto others.
Pretty good, except for... - List software tagging the subject line... consumes space available for actual subject content and is redundant... this is bad. - Crossposting... has inclusivity purposes and is automatically reference efficient while permitting further joint or local discussion under that... not as universally bad as people claim. Like those refusing to edit their own body etiquette, such claimants are really saying they're too lazy to edit their own headers as may be prudent when they post their occaisional reply to a crosspost. If they don't reply, relavant crossposting is irrelavant.
create filters
Filters don't fix the errors of friends you should be helping in the fight. Mailing etiquette exists to strengthen comms. If your friend is addicted to crack, you'd help. Bad etiquette is crack and weakens your game. Accept the help of your friends, break the habit, up your game. As in language, common denominators in etiquette make it easier for people to communicate, and to do so efficiently without undue parsing cycles, interpretation, and loss.