Make authors's e-mail address visible for everyone

List of all postings - shows name, location, but the e-mail address is missing,

Simplest way to add the e-mail address?

The simplest solution would be to put the email address in the bio which can be done from the Staff users profile (Settings → Staff).

Yes this is possible, but a mailto link can not be inserted.

I tried to modify the author.hbs file in this way

{{#if mail}}

but this not possible - the “internal notification” mail variable isn’t available in this context.

That’s correct, it would have to be a text inclusion in the bio, and it’s not possible to pull in the email associated with the staff user with helpers.

If you have a set number of authors, another thing you could do is create custom author templates for each specific author: Author Context: Ghost Themes - Documentation

That way, you can add the author emails as mailto links directly using HTML in each template file.