In order for Gmail to send mail from an email program like Outlook or Thunderbird, the program needs to understand how to communicate with Gmail's email servers. It does this via Simple Mail Transfer Protocol (SMTP) server settings. The settings are the same for every email provider you use with Gmail.
How to Set Up an SMTP Server in Gmail (EASY!)
When you set up an email client to sync with your Gmail account, a screen will appear asking you to enter your Gmail SMTP details.
SMTP settings are only for sending emails; you must also specify settings for receiving emails. Receiving emails is done via POP3 or IMAP servers. Before identifying these settings in your email client, enable access via settings in Gmail by going to Settings > Forwarding & POP/IMAP.
Before you continue troubleshooting, remember that you are entering the wrong password. You can reset your Gmail password if you forget it.
