There's a remote based colleague who has one of the worst email etiquette I've ever seen. He frequently initiates emails with all the text (usually 100-150 characters) in the subject. In email chains, he frequently fires off 5 emails with one or two words in each of them.
Really, the way he uses it is perfect for Slack, but horrendous for everyone's inbox. I've tried to get him to at least look at Slack by creating a channel for a specific project and asking him explicitly to use it, but he didn't even acknowledge me and continues to use email in this manner. Everyone else involved in the project also tried to give him the hint by using the channel and replying to his emails with "We mentioned that in Slack, have you seen it?" but he still has not acknowledged the existence of it.
I know it's not a huge deal like bad smells or loud noises in the office. However this has gotten annoying enough and gone on for long enough that I want to personally confront him. If I said something very direct like "Sending multiple one word emails in succession clutters everyone's inbox. Instead, let's use Slack for communications of that kind from now on. (Insert link to company's guide for installing Slack app and logging in)." Would that be too harsh or unprofessional?