There is an old saying when it comes to HTML. If you touch the code, it is broken and must be tested. All you need to do is miss a quote or miss a closing > and the entire message will not display. Good luck finding the needle in the haystack.
The big advantage of the WP-Omeda Omail plugin is that you never have to touch any code. That means that the job of creating and sending the newsletter can be managed by a person who know the content and the advertising and does not need to know a thing about HTML code. Instant productivity.