Simple Mail is a Firefox extension that can turn your favorite web browser into a basic, yet fully functional email client.
Once installed, you can configure Simple Mail with your email account details and manage your mailbox from just another tab in Firefox.
The extension works with POP3, SMTP and IMAP email accounts – no webmail support here, but hey, simply open another tab and browse to your webmail page.
Also, you can set up as many simultaneous accounts as you want.
Simply Mail behaves just like any regular email client: you'll be able to check your messages, reply to them, see any attached files and of course send new messages from a full featured rich text editor.
The program also includes some nice options such as an address book or the possibility to download just message headers. Just bear in mind that this is meant to be a simple, lightweight client so there won't be extra features like signatures or spam filters.
If you only need a basic, simple-to-use email client for your POP3 email account, then Simply Mail can be a perfect solution, as long as you use Firefox as your web browser.