One of the first things I noticed when I started using CampaignMonitor was that my HTML emails degraded really well when forwarded.
Typically, unless your HTML mail is simply tables & images, standards-based HTML formatted email degrades very poorly, stripping away CSS and generally making it look crummy. Email clients do this to varying degrees, and some are worse than others, but generally you need to use a "Redirect" or "Send Again"-type function to keep their format together.
One thing about emails from CM, the HTML part comes out fixed-width of ASCII characters per line, and with all whitespace stripped. I think the fixed-width part is pretty standard, but the whitespace stripping is new to me.
I wrote a new template when I started using CM, so it could very well be that too. Have I accidentally stumbled on the secret to forwardable HTML-email design? Or does CampaignMonitor sprinkle some magic fairy dust over outgoing mail to make unbroken forwarding possible? If so, what is this magic fairy dust, and where can I get some?
You might also check for an option to "redirect" your newsletter, which forwards the email to someone else verbatim... just as it came to you. Not all apps have this option, but for those that do, it's nice.
Can't edit the original newsletter content, but I don't see why anyone would need to anyway.