Which Email client used is set on a user by user basis. A user would need to edit their profile and set their Email Client to External Email Client.
If it is happening to all users and the External Email Client option does not appear in the dropdown, it may have been inadvertently removed. To rectify this please do the following
Go to the Dropdown Editor, within Studio
Search for the dropdown list "dom_email_link_type"
If you only see one entry for Access CRM Email Client then add the following: Item Name = mailto, Display Label = External Email Client
Click Save
Now each user will need to go into their profile and edit the above mentioned option.