Not happy with the Mac's mail program? No problem. Here are three very good alternatives.
This unique mail program is my go-to one. It's unique among mail software, because it creates a unified inbox from all of your mail accounts, and lets you read, create and send mail from them all without affecting the mail on their servers. It stores all of your account information, but not your mail, on its servers, then delivers mail to you. It's ideal for someone like me who has multiple accounts and uses multiple computers, because that way, your mail is never only on one machine --- you can read it everywhere. It doesn't quite have the bells and whistles of for-pay mail programs, but for me it's perfect. It may well be that way for you as well.
Thunderbird has been around a very long time, and it's much a solid choice today as it ever was. It's open source, gets continually updated, and bristles with features. You can read emails in separate tabs, customize the way it works, use plenty of free add-ins, and more. It has a nice complement of search tools, and you can also search the Web from inside it as well. It's beginning to look a bit long in the tooth, but still, it's worth a try.
Opera doesn't just make a browser -- it makes a mail client as well, and it's free. Like the Opera browser, it's lightweight and requires little RAM or system resources. You can view messages in their own tabs, and sort mail easily. It's also got a nice RSS reader build into it as well.