Postbox is a free email client for Mac which helps in getting things done through the application, more than just managing, organizing and reading emails. Apart from that, Postbox has lots of options for users to make it a complete email reader, manager and organizer. My opinion, its better than mail.app default for mac.

Postbox has something more than just an email client, and works perfectly with no heaviness in Mac OS X.
Here is a list of features you find with Postbox –

• Powerful search options – You can search through mails in your own way. Search the mails from particular user, to particular email address, before or after any particular date, search for specific words in the email subject or body.
• Searching through tabs – Search tabs are used to find through attachments, images, links and contacts and in this, there is no involvement of the message body sections. You can create a new message directly by using the found searches.

• Contacts section – Use the Contacts Panel to send an email, dial a phone number, initiate an IM chat, or lookup an address. You can even search for recent messages, attachments, images, or links from any contact, anytime.
• Tabbed Mail Browsing – Similar to a web browser, mails can be viewed in separate tabs, so you don’t need to close one tab when viewing another mail. You can switch through the tabs while mailing, and customize and organize things effectively.
• Focus by Topics – You can organize the mails in various topics, and viewing mails in specific topics and make things pretty easier. Eg. you can organize mails for project, for an event etc.

Many other features include tagging, archiving, editing, web composing, integration of web search, anti-phishing and malware services, and automatic updates.

You can download the Postbox email client for Mac OS X from Postbox-inc.com!