Mozilla Thunderbird - Gmail
Mozilla Thunderbird is a specific open source mail client and RSS reader made by the organization who produces the Mozilla Firefox web browser. This is a free email client you can use with Gmail account.

Mozilla Thunderbird is a cross-platform, open source email client that competes with the likes of Microsoft Outlook and Eudora Mail. This project strategy was modeled after that of the Mozilla Firefox web browser. It is installed by default in Ubuntu desktop systems.
December 7, 2004, version 1.0 was released, and received more than 500,000 downloads in its first three days of release, and 1,000,000 in 10 days.
Thunderbird is a computer program and is produced in unique versions for Windows, Mac-OS and Linux.
On December 1, 2015, Mozilla announced in a company-wide memo that Thunderbird development needs to be uncoupled from Firefox. Thunderbird developers are putting in huge efforts in responding to changes to Mozilla technologies.
Why Thunderbird?
It provides enterprise and government-grade security features such as TLS/SSL connections to IMAP and SMTP servers. And also offers native support for S/MIME secure email. Other security features may be added through extensions. For instance, PGP signing, encryption, and decryption.
Thunderbird supports add-ons, extensions that provide additional functionality to the e-mail client. Single of the key extensions available for Mozilla Thunderbird is Mozilla Lightning, an extension that adds calendar and personal information manager (PIM) capabilities to Thunderbird.
Features
Message management - Thunderbird can manage multiple emails, newsgroup, and news feed accounts as well as supports multiple identities within accounts.
Junk filter - Thunderbird incorporates a Bayesian spam filter, a white-list based on the included address book, and can also understand classifications by server-based filters such as Spam Assassin.
Extension and theme - Extension allows the addition of features through the installation of XP Install modules. Thunderbirds support a variety of theme for changing its overall look and feel.
Standard support
POP - This enables you to move your mail from remote email server to a local folder.
IMAP - IMAP itself has many capabilities, and Thunderbird selected many of them, plus added their own as well as included the de facto standards by Google and Apple.
LDAP address completion.
S/MIME Provide encryption and signing, relying on X.509 keys provided by a centralized certificate authority.
File formats supported
m box – Unix mailbox format (one file holding many emails)
mdir – known as maildir-lite (one file per email).
Steps to Configure Mozilla Thunderbird:
Start Thunderbird.
On the Tools menu, click Account Settings. The Account Settings dialog box appears.
Now click on Account Actions, and then click Add Mail Account. The Mail Account Setup dialog box appears.
In the below image type your name, email address & password.
If you do not want to retype the account password every time you start Thunderbird, select the Remember password checkbox.
Click Continue. It automatically detects the mail server settings for the account. If Thunderbird is unable to detect the settings, follow steps #7.
7. Next Steps are :-
Select IMAP server - imap.gmail.com & Select Port - 993.
SMTP server - smtp.gmail.com & Select Port - 465.
Select SSL/TLS in both server (imap as well as smtp).
Select Normal Password Authentication in both server (imap as well as smtp).
8. Click Done. Thunderbird verifies the password.
9. At last, in the below image you can see that Thunderbird has been configured successfully.