The hack

You can send a Teams message to anyone if you know their email or phone number.

Chat with any Slack user
Thoughts, stories and ideas.
First Thousand CustomersJan Tovarys

Why it works

Everyone lives in their Teams workspace. Probably even more than they do in email.

Compared to Slack, there is no double-opt-in to accept the invite. Here it's show directly as messages, which is even more effective.

How to do it

Go to your Teams account and search using either email or phone number and send them a message

There are two possible issues with this depending on who you try to reach:

  1. There are message limits in place: messages prior to an explicit ‘accept’ are limited to 10. Additional messages are not delivered.
  2. Companies can have this feature turned-off

Read about all the details in the official Microsoft post.