BuddyPress.org

Hi,

When a user sends a message to another user on my site, the recipient gets a notification via email telling him/her that there is a new message.
The email has a link to the ‘discussion’.

If the user clicks on the link and is not logged in on the site, it goes to a 404 page.
How can I show a login page if the user is not logged in and redirect to the discussion later on?

Thanks