To re-verify your client's access via the web to a Gmail Account.
- First sign into the Gmail account in a browser on the device where you are setting up your client
- Go here and enable access for "less secure" apps: https://www.google.com/settings/security/lesssecureapps
- Then go here: https://accounts.google.com/b/0/DisplayUnlockCaptcha and click Continue.
- Then go back to your client and try again
However, if you use 2 step verification on your GMail account, don't forget that you will also need an application specific password for your client/device. See http://support.google.com/mail/bin/answer.py?hl=en&answer=1173270 for how to do that.