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Chrome
In Chrome, click on the three dots in the top right corner to access the menu and select Settings.
Select Privacy and Security, Site Settings, and then Additional Permissions.
Click Protocol Handlers.
Select Sites can ask to handle protocols
Double check to make sure that Gmail doesn't appear on the list as blocked. If it is, remove it from the list.
Open Gmail in Chrome.
Look for the two diamonds in the far right of the address bar and click on it. Choose allow mail.google.com to open all email links.
For Firefox, Edge and Safari, follow these instructions.