Another email client, other than the previous post Yahoo! Zimbra, has come into my mind. I forgot that aside from the great browserFirefox, Mozilla has also another project which is Thunderbird.
Thunderbird is an email client which is offered free from Mozilla Org. Against Yahoo! Zimbra, this browser is fast.
Mozilla Org. is continuously updating this project and now it has this features:
Tabs and Search
-Tabs allows you to load emails in another tab just like Firefox's Tabbed browsing.
-Search allows you to easily look for the email you wanted to view using filters and etc. just to pinpoint exactly the email.
Message Archiving
-Archiving allows you to store emails in a folder system. You can store emails that you feel you need it in the future then take it out from the inbox and so archive it.
More more features of this email client visit here.