(hyper?) LInks suddenly stopped working

I have an iMac running High Sierra 10.13.6. The other day links, which should open an email in Apple Mail with the recipient's address in the top line, started sending me to Google instead. If I click the link on another Mac (or in webmail) it works fine. The same happens with Chrome (my default) or Safari.

How can I get the links reconnected to Apple Mail?

Any help would be appreciated.

John