Enable access for less secure apps:https://myaccount.google.com/lesssecureapps
Turn off 2-factor authentication:https://myaccount.google.com/signinoptions/two-step-verificationnebo Leave it on, but create an application password for SingleCase as instructed here;
Turn off captcha:https://accounts.google.com/b/0/DisplayUnlockCaptcha
Now you can start sending and receiving emails from Gmail.
Note: You may receive the message "This setting is managed by your domain administrator":
If this is you, visit the admin console at admin.gogle.com and under Security, Less secure apps, select "Allow users to manage their own access to less secure apps" and click "SAVE".
Then repeat step 1.