If your Gmail account has an additional layer of security on it, you need to follow these extra steps to make your Gmail SMTP work.
Creating App Password in Google
As Clientjoy is not a Google App it will be recognized as a "less secure app". That doesn't mean that Clientjoy isn't secure, it is just what Google calls third party apps. So in order to connect your Gmail account securely, you would have to create an app password, here's how
Login to your Google Account.
Click on your Profile photo on top right.
Click on Mange google account.
Choose 'Security' on the left navigation panel.
On the 'Signing into Google' panel, choose App passwords and copy the App password.
✍️ Note: If you can't see app passwords in the Gmail Account, ensure that the 2-Step Verification is enabled and set appropriately for your account. See below instructions to enable it.
Enabling 2-step verification in Google account and creating app password
In the security tab only you'll see the status of your 2-Step verification as shown in the image below. Click on it.
Complete your 2-step verification setup as guided by google in next screens
Once finished the setup, turn on the 2 step verification.
Now click on the app password option, generate new one and copy the app password.
You need this password in the Clientjoy SMTP settings.