Granddad of all email clients in Linux. It supports a wide range of protocols apart from being the most complete mail client. This mail client also boasts the best integration with the GNOME desktop. Synchronising Gmail accounts for address book and calendar operations are a breeze. Evolution has the best email account assistant when it comes to Google accounts. MS Exchange 2003-2010 accounts are also supported via evolution-exchange and evolution-mapi plugins.
However, setting up exchange mail boxes are not that easy and are mostly a hit and a miss. Last time I checked, if the exchange server is behind a proxy, you may not be able to get it working at all. Apart from that, the other big drawback of Evolution is its performance while downloading emails. Evolution is painfully slow when it comes to downloading emails. Over the years it has improved but its still very sluggish compared to Thunderbird.
Evolution only works in Linux, a Mac version is also available but is not updated regularly. If Evolution can improve on the speed, it will be a tough call. Apart from Ubuntu most Linux distributions ship Evolution as their default email client. Evolution comes second in my list of email clients for Linux.