Don't send newsletters with multiple columns - like you don't design pdf's with multiple columns. These are only good for reading as a print out. Reading either online is exasperating, since you can't scan these easily.
Newsletters sent in email are patterned after a text email, only with links at the top so you can quickly get down to what's interesting to you. A sort of Table of Contents at the beginning.
And on sites - you want search engines. "As ye surf, so shall ye design..." or something like that. People use search engines to get what they want - quickly. The Internet is built on speed, choice, information. Those three.
So you email newsletters need to encompass these. The only exception would be a good narrative, a good story that pulls the reader along. When you skip around in a story, you lose track quickly. Best, then, would be mini-stories connected with in-text links.
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Sent this out as I just unsubscribed to a really chock-full newsletter that I couldn't find anything in it. Looked like a newsletter - but without the printed page. Lost something in the translation. Just like those (idiot) pdf's that people figure you are going to print out.
I design in 6x9 format, so that you can put two up on a page and read it simply. letter-sized pages require really big type to read on the screen - or lots of paper to print out. 6x9 still gives plenty of space for notes - but can be printed two-up for easy reading either way.
Keep the user in mind - always.