In addition to the other answers, a layout that works with minimal technology assumptions -- small screen, images off, accessibility options (like large fonts) on, etc. This is partly about liquid design but also avoiding elements that are almost guaranteed to cause problems: multi-column layouts, tables, anything with hard-wired size or colors.

A way to get a plain-text version, in case you fail at any of the above.

Relevant email addresses, particularly opt-out, in the body of the message and not just behind a "click here" link.

In addition to the other answers, a layout that works with minimal technology assumptions -- small screen, images off, accessibility options (like large fonts) on, etc. This is partly about liquid design but also avoiding elements that are almost guaranteed to cause problems: multi-column layouts, tables, anything with hard-wired size or colors.

A way to get a plain-text version, in case you fail at any of the above.

Relevant email addresses, particularly opt-out, visible in the newsletter, not hidden behind a "click here" link. (If I print the newsletter to read on the internet-less bus, when I get to work I should be able to read the RSVP address for that event that's coming up so I can send email directly.)