I'm looking for a good, lightweight email client
Popular email clients for the Mac include:


Apple Mail - my current favourite, especially in Lion.
Postbox - Paid for, but feature-rich and quite polished AFAICT
Thunderbird Free and open-source. Extremely configurable with a similar extension ecosystem as Firefox.
Microsoft Outlook - A very fully featured email and calendar package. It replaces Microsoft's previous Mac email client, Entourage.
Sparrow - A unique email application. The layout encourages casual, Twitter-like conversation by evoking the timeline view of popular Twitter apps. It's pretty trendy at time of writing.
Mailplane - Basically an optimised, dedicated Gmail browser window. Worth looking into if you're a heavy Gmail webmail user.
MailMate - I hadn't heard of this one before, but it has a good review at Lifehacker.