While Apple Mail for OS X is in widespread use and generally popular, it hasn't stopped developers from coming out with competing products that add to or extend the Apple Mail feature set.
Postbox and Mail Pilot are a couple of good options, and Airmail, which I'm looking at today, has had good reviews as well.
Airmail just released a big update to version 2.0 (US $9.99 in the Mac app store) that has added some very Yosemite-friendly features like iCloud Account sync, iCloud attachment, upload and link sharing, extensions to send mail from other apps and the ability to reply to messages from a notification banner.
The app supports many mail protocols including Exchange, iCloud, Gmail, IMAP, POP3, Google Apps, Yahoo!, AOL, Outlook.com, and Live.com. I was especially happy to see POP3 legacy support, as many third-party mail apps are dropping it.
I gave the app a try, and it quickly imported my Apple mail account. The design of the app is very clean, and I liked the integration with Apple's Calendar, Reminders, and other apps like Evernote, 2Do and Omnifocus. The app also supports a variety of plug-ins, and has a menubar icon. Apple, are you listening? Attachments can link to Google Drive, Dropbox, CloudApp, Droplr and FTP.