Great thought, I have several thoughts about this.
It's great to have a goal of making newsletter and marketing emails look personal. Merging in the customer's name and keeping a personal tone are great ways to do this.
If done professionally, bold and color headlines shouldn't detract from the professionalism of an email, they should just make it easy to scan, which I find that most readers appreciate.
As an example, I subscribe to several stock newsletters. They use headers to clearly delineate categories, stocks, etc. I always appreciate that, because I don't want to spend a ton of time reading the whole newsletter, I want to quickly jump to the items I care about. But it's done in such a way that the email remains conservative in feel.
As far as emails looking "salesy", I often hear this from people like it's always a bad thing. If you start a newsletter that's informative, then you clearly want to respect your recipients and not turn it into a sales email. But for many businesses who are emailing actual customers, sending marketing type emails is the right thing to do. When I get my emails from Nordstrom, I expect them to send me an ad about their new products, sales, etc. I'm not expecting a newsletter from them. The same holds true for many businesses emailing their customers.
Of course whether your email falls into this category or not is a question you have to ask yourself.
If you're concerned about degrading the professionalism of your newsletter, maybe try mocking up a version with headers and have a few friends, colleagues, or even customers review it and tell you what they think.
Now, if I could just get my Mom to add headers to the emails she sends me, then I could skim those too!