Slack is the poor mans IRC. The Dropbox of messaging apps. It's easy but if you want to hide your email from public view or customize anything good luck. Slack is notoriously ignorant to customer requests.
Whenever someone compares Slack to IRC I try to imagine my company, where 70% of people are non-technical, using some IRC client as our chat app. There's no way this would happen.
Slack was never meant to replace IRC, it was meant to be used at work. Why would you hide your email from your colleagues?