Why it works
Referral programs fade the moment they stop being mentioned. A monthly nudge keeps them alive, and because it points at your careers page — which you keep current — the same frozen email always surfaces today's open roles. That's the "static email + updating link" pattern doing the work: the message never changes, the roles behind the link do.
Make it yours
Point at a live careers or jobs page so the roles are always current without touching the email. If you're pushing a specific hard-to-fill role, add a line calling it out and refresh that line when it changes.