It seems that if you use an email address that contains a ‘#’ character and is part of a mailto tag, then clicking on it on the browser will open Outlook, but will not add the complete email address in the ‘To:’ field; it adds all characters until the ‘#’ character.
To overcome this replace the ‘#’ character with %23
So, if you had an email address like: #projectTeam@company.com the mailto tag should have it as %23projectTeam@company.com