场景

1、注册了新的Gmail 邮件帐户
2、配置Thunderbird 客户端无法连接，要么出错，要么说用户名或密码错误，后来发现是IMAP服务没启用，于是登录web启用该设置
3、登录Gmail web邮箱，收到account block邮件，按邮件内容操作重新设定安全级别，Thunderbird客户端仍然无法登录Gmail
4、在客户端连接Gmail出错时弹出tool tips，于是按google support去查找答案
5、 进入 http://www.google.com/accounts/DisplayUnlockCaptcha 时有个启用APP进入的权限按钮，把权限放开了，客户端就能正常连接到gmail了
6、总结来看，是Gmail目前的安全级别提高了，

参考：
http://support.google.com/mail/accounts/bin/answer.py?answer=78754 或
https://support.google.com/mail/answer/78754

My client isn’t accepting my username and password

If you’re repeatedly prompted for your username and password, or if you’re getting an ‘invalid credentials’ or ‘web login required’ error, make sure your password is correct. Keep in mind that passwords are case-sensitive.

If you’re sure your password is correct, you can try these troubleshooting tips:

• Sign in to your account from the web version of Gmail at https://mail.google.com. Afterwards try accessing your messages in your mail app again.
• If you’re still having problems, visit http://www.google.com/accounts/DisplayUnlockCaptcha and sign in with your Gmail username and password. If necessary, enter the letters in the distorted picture.
• Your mail app might not support the latest security standards. Learn how to allow less secure apps access to your account.
• Make sure your mail app isn’t set to check for new email too frequently. If your mail app checks for new messages more than once every 10 minutes, the app’s access to your account could be blocked.
• Try changing your password according to our tips on creating a strong password.