If you are typing email addresses in Google Sheets, you have to make sure that the data you entered is correct. The easiest way to do this is by confirming that the email address format is correct. You can do this by Data Verification. First, select the cells where you want to apply data validation. Then, select Data → Data Validation from the navigation menu. Then you will see a small window you have to fill in. Make sure the Cell range is correct and choose 'Text' and 'contains' for 'Criteria.' Next, enter the @ symbol in the box next to 'contains.' Next, for 'On Invalid Data' you can either choose 'Show Warning' or 'Reject Input.'