Context
When you initially open Gmail in Chrome, a protocol handler icon 
may appear in the omnibox next to the star bookmarks icon proposing to allow Gmail to open all email link (=mailto: links).
You might have answered No or Ignore at first and want to change this answer.
Solution
When opening Gmail, a protocol handler icon may appear in the omnibox next to the star bookmarks icon, click on it to change you decision regarding email links handling.
More details.
