are protected under copyright law.
If you copied a copyrighted work without authorization, you're infringing. Whether that's unlawful depends on whether you can mount a successful defense. One possible defense is that you are a co-owner because you designed the template used in the newsletter. Proving that will be an uphill battle because common elements of web designs are not protected by copyright --- font choices, look and feel -- and because many of the design choices are limited by practical realities and should be free for everyone. If you can overcome these presumptions (and there are no written work made for hire agreements or assignments), you might be able to claim co-ownership. You might also be able to claim fair use considering that your use was for portfolio purposes and did not cause the owner a loss in revenue. Because co-ownership and fair use are expensive to prove (only a judge or arbitrator can decide), many graphic and web designers deal with these issues in their contracts by including a "portfolio provision" or by retaining non-exclusive rights for certain design elements.