A while ago I got frustrated with a number of Apple Mail limitations the most severe being that you couldn’t prevent HTML from automatically rendering when it displayed in the preview pane. Automatically rendering HTML could let malicious web code load just because you had your prview pane open and accidentally viewed a message. I looked for an alternative and found Postbox to be a great one. It has all the features you’d want from an email client and more and is very security conscious supporting the ability to not load HTML content unless specifically instructed to. It’s a real power user email client with tons of advanced ways to manage emails lke assigning and viewing by topic, filtering by various criteria, advanced filtering rules which are akin to Outlook rules, precanned responses with variables, and more. While it's a paid app the price ($32 as of the writing of this entry) for what you get is very reasonable. They have a PC version as well. Check it out!