It's not free and not that cheap, BUT is has tons of useful features for a Mail user and the developers add something new on a very regular basis. I use the new Tasks feature all the time. So highly recommended if like me Mail is one of your main communication tools. Also check out MailHub which is my other essential and MailFollowUp
Even when you have a subscription, they plaster your mail app with ads for their business plans. Thet's ridiculous. They promised new features but magically these are only added to the business plan that's 3 times the price. That's not how you value your customers.