FYI: I’ve been reporting this (email signatures appearing in posts) for several months now and providing sources of raw emails, but unfortunately no fixes have been discovered and/or successful yet despite efforts. 
I don’t think trying to increase the portfolio of already-brittle email-in functionality is the right approach here. Rather, we should be trying to get people on to the web site to take full advantage of what Discourse has to offer.
While reading the code, I discovered some lines that suggest that this feature has actually implemented.
https://github.com/discourse/discourse/blob/master/lib/email/receiver.rb#L250
This feature is actually much more complicated to discover. 
Yep, it was implemented as part of the MOSS grant, relevant thread here:
Yay! Can’t wait until this is exposed through the email UI. 
There are no plans to expose this in mail UI – you’d need to Admin, Customize, Text Content
Understand that it’s something to be added to the template regardless, but my anticipation is more for this as a first step toward the proposed behavior in the RFC (clicking) more so than the like-by-reply.
How? what should be added?
Something to the effect of "You can like things by replying to this email with the text ‘Like’ (localized to the instance’s language), or ‘+1’ "
Hi, I’m re-opening this topic to continue the conversation about exposing the +1 in email. What do you think about putting it in this error message instead?
See, my reply message was too short only because I was trying to say, “Got it. Thank you!” It didn’t make sense for me to make that longer, so after some research I discovered what the Like behavior was for. But then I thought, “How do I Like via email?”, which finally took me to this thread.
I imagine that other people whose reply was too short were also trying to “Like” a post in their own way, in which case instead of just having “If you can correct the problem, please try again”, you can also say “If you were trying to say “Thank you”, “I agree”, or another term of gratitude or agreement, you can reply with +1 to Like the post instead.”
This is a great suggestion, @Osama can you improve the text here.
Instead of Body is too short (minimum is 20 characters)
Your reply was too short, minimum length is 20 characters. Alternatively, you can like a post via email by replying with ‘+1’.
@codinghorror is that text OK? I am really not a fan of the “Body is too short” text anyway cause it is way too … computery.
We could get very “instructive” here …
To promote more in depth conversations, very short replies are not allowed. Can you please reply with at least 20 characters? Alternatively, you can like a post via email by replying with ‘+1’.
It’s not
We hate your short reply
so much as
We love actual conversations
I think the idea of communicating
We love actual conversations
Is exactly right!
Us too but we love sleeky conversations too.
Having a discussion blooded by thanks, me too etc. makes it less readable, getting to the bottom line.
So, is there an answer to
Is this actually implemented? Is it documented as such? Are there other “magic” replies?
Yes, as it is said on this own topic a few replies above
https://github.com/discourse/discourse/blob/master/lib/email/receiver.rb#L997-L999
No.
So, no. 
Hmmm. Some site’s docs like those on GNOME discourse suggest that watch, track, and mute will work. Is this from some plugin they are using?
I read that your were asking for other types of “post actions”.
For topic level actions there are those:
https://github.com/discourse/discourse/blob/master/lib/email/receiver.rb#L1018-L1032
Oh wow! I didn’t know about the mute track watch email replies. Maybe a topic in the Tips & Tricks category about how people can take topic / post actions via email would be a good documentation step?
I kind of feel like this is such a power feature… and in most communities I’d assume the desire is to get people in the habit of logging in to perform those actions. It might be nice to document this somewhere but I wouldn’t advocate for it being especially prominent.
Do note that there is an unsubscribe link at the bottom of all emails from discourse. If you select that, you get prompted to stop watching the specific topic you are being notified about or to unsubscribe from all email from that site.
