The template
Subject: Broken link in your {post topic} post
Hi {first name},
Quick heads-up: in your post on {post topic}, the link to {broken resource name} under "{section or anchor text}" goes to a 404 page now.
If you're looking for a replacement, our {resource title} covers the same ground: {URL}. No worries if you'd rather point it somewhere else.
Thanks,
{your first name}