If you set up Gmail to receive alert and get an error " failed to send email", that would be your email settings cause the issue.
Please kindly refer to the steps below to fix the issue.
1. If you have turned on 2- Step Verification for your Gmail like the picture below.
You can refer to the link How to Generate an APP Password in Gmail Email Account to generate an app password and use the app password as the email password to make the email test.
Then use this app password as your Gmail password.
2. If you never turned on 2 step-verification, you can refer to the steps below to turn on the" Less secure APP access" option.
Step 1.Log into your G-mail account and go to Google apps -> Account-> Security page.
Step 2. Scroll the page down to Less secure app access and click to turn it on.
Step 3. Allow access to your Google account.