This is not nopCommerce issue, all you need to enable 3rd party application to access your gmail account. Follow this steps to fix this:
1. Login to your gmail account.
2. Visit this page https://accounts.google.com/DisplayUnlockCaptcha and click on button to allow access.
3. Visit this page https://www.google.com/settings/security/lesssecureapps and enable access for less secure apps.
4. Login to your nopCommerce admin portal and visit this page http://website_name.com/Admin/EmailAccount/List and click edit and click to delete the account.
5. Now use following details:
Email address:
[email protected] E display name: your website name
Host: smtp.gmail.com
User:
[email protected] Password: password, remember to click change password button if you are editing any existing account
SSL: tick to select it
Use default credentials: uncheck, do not select it
6. Click on save button above.
7. Test the mailing by entering your email id and click on send test email button.