I took a look at the newsletter in question here and it looks like the reason that the mailing was breaking was due to quotation marks in the names of some of the links.
Say for example I call one of my links "link1" using the Link name field below, when the HTML is compiled that name ends up in double quotation marks as ""link1"" which will essentially cancel each other out and break the other formatting tags. In the "Link name" field below, it's best to avoid using quotation marks.
I've left a support copy of the newsletter in your account where I've removed the quotation marks from the names of the links and found that there were no rendering issues. So feel free to use that if it saves you time (but be sure to test yourself!).