If you forgot your mail password and it’s stored in your mail client you can use MailPass view to reveal it.
To do that you need to download MailPass view from it’s official link:
Mail PassView is a password-recovery tool that reveals the passwords and other account details for the following email clients:
• Outlook Express
• Microsoft Outlook 2000 (POP3 and SMTP Accounts only)
• Microsoft Outlook 2002/2003/2007/2010/2013/2016 (POP3, IMAP, HTTP and SMTP Accounts)
• Windows Mail
• Windows Live Mail
• Netscape 6.x/7.x (If the password is not encrypted with master password)
• Mozilla Thunderbird (If the password is not encrypted with master password)
• Group Mail Free
• Yahoo! Mail – If the password is saved in Yahoo! Messenger application.
• Hotmail/MSN mail – If the password is saved in MSN/Windows/Live Messenger application.
• Gmail – If the password is saved by Gmail Notifier application, Google Desktop, or by Google Talk.
Some antivirus software can detect Mail PassView tool as infected with Trojan/Virus. You can find more information on the official site.