This is common on many sites, but using the Moodle User Association an an example:
If I get a forum post by email, and I click on the "See this post in context" link that goes to somewhere like https://moodleassociation.org/mod/forum/discuss.php?d=695, then that immediately redirects me to https://moodleassociation.org/user/policy.php which shows an unfiendly page saying just "Site policy agreement".
Surely we can do a lot better than that, but putting a poilite call-to-action at the top of the page. Something like:
"Sorry to bother you, but before using this site, new users must agree to our policies. Existing users: please log in."