I think one of the article's point is precisely that maybe that one email that is "Not colorful, but also not incredibly encouraging", gets to you when it's the Nth one you get, for different values of N for each person.
And the reason the email "gets" to you is because you might be in a bad mood, you might not love to wake up to a pile of not-so-nice emails, you cat scratched your favorite sandals today, etc, but in the end the author's point is precisely that maybe the other side of the communication channel is having issues of its own as well, and thus responding very poorly to the situation ("stop sending what I consider spam but you consider business"), i.e. when he says "give people the benefit of the doubt and respond with kindness and empathy regardless of the way other people are interacting in the situation"
And the reason the email "gets" to you is because you might be in a bad mood, you might not love to wake up to a pile of not-so-nice emails, you cat scratched your favorite sandals today, etc, but in the end the author's point is precisely that maybe the other side of the communication channel is having issues of its own as well, and thus responding very poorly to the situation ("stop sending what I consider spam but you consider business"), i.e. when he says "give people the benefit of the doubt and respond with kindness and empathy regardless of the way other people are interacting in the situation"