Since it is better to be safe than to be sorry, I recommend you to test as much as you can before sending out your newsletter or email marketing campaigns. Why do I tell you this? Because it is the only way you can make sure that your clients will receive a perfect newsletter from you. Always test your links, make sure that the images were uploaded correctly, and make sure that everything functions perfectly.
I also strongly suggest that you create a list with five contacts, it can be people from your office , and this way, ask them to look at the newsletter and everything is functioning perfect.
Lastly, I also recommend that you send your newsletter to a first group (around 20% of your list) and test how effective the newsletter is. If you want do a survey on what they liked from the website and what they did not like. That way you can act for the remainder of your clients based upon the first 20% feedback.