Most orgs have an internal SMTP server that will accept and send mail to other internal addresses without any special authentication or validation. It’s almost essential for automatic monitoring software and that sort of thing.
Where the barriers go up is at the border to the Internet. And thank goodness, just a couple decades ago it was sheer chaos.
Mastodon:
Open it in your web browser, follow the prompts to sign up.
Search for people you want to follow, and add them. Maybe search for subject matter that interests you, and follow people who post about that.
PixelFed:
Open it in your web browser, follow the prompts to sign up.
Search for people you want to follow, and add them. Maybe search for subject matter that interests you, and follow people who post about that.
Is that essential, though? As long as everybody is federated, it shouldn’t matter, except maybe it will take longer to see some posts.