This is simply a zine delivered through e-mail. They're often free of charge. Many do accept feedback letters and commentary, but tend to be much shorter than print zines - so the letter section is also shorter.

Example E-zines:

There are many different themes and varieties of Ezines. Here are some to start with.

Boogie Jack: This is mostly targetting Web Design, but also covers some life events. The newsletter is called Almost a Newsletter.

Sitepoint: One of the most detailed Web design portals. It also offers several design-themed newsletters.

Writing Etc.: Filbert Publishing sells a wide range of publications for budding writers. This newsletter covers many such avenues.

Writers Write: More of a listing zine, but very encouraging for writers.

Heroic Stories: Want to hear stories of the good ordinary people do? then this newsletter's for you.