I don’t use a lot of email testing tools, but when I do, it’s usually one of these.
Let them help you in times of need, here we go:
Mail Tester – It’s a great tool to quickly inspect in what condition your email deliverability is and to see the spammyness of the email content/structure
Putsmail – Love to use this one when I want to quickly test out the rendering of HTML emails in different clients. Pop the code in, send to your email accounts and check the rendering, quick and easy.
Hemingway App – This one is great to do a quick test on copy readability, just making sure it’s all easy to understand.