Users need to verify that your settings match the settings specified here. If your Email Alerts settings are correct (Step 1), you need to enable Gmail’s access to less secure apps (Step 2).
Step 1. Check Your Email Alerts Settings
The following screenshot shows how your Email Alerts information should be configured.
Step 2. Enable Access for Less Secure Apps in Gmail
If that doesn't solve the issue, a firewall may be blocking traffic. Please contact us for more information.
Note: Gmail has a limit of 500 emails that can be sent or received in 24 hours. If your amount exceeds this number, Google may flag your account as a spammer and lock it.