> if you deal with anyone outside of your own employer email is still the standard for communication

I get what you're saying and agree email is almost always the least common denominator between two different organizations.

On the flip side, this can really vary based on the relationship between two orgs and how closely they might work with each other. I've definitely had Teams instances with outside users and Slack channels shared between multiple orgs when there's a lot of close daily collaboration happening.

https://slack.com/blog/collaboration/slack-shared-channels

https://learn.microsoft.com/en-us/microsoftteams/communicate...