The 21-day clock on a DPN starts when it is issued, not when your client opens...

https://dlf-ne.org/how-do-i-spot-a-lockdown-dpn-before-the-client-wastes-money-on-the-wrong-step/

The 21-day clock on a DPN starts when it is issued, not when your client opens the mail. Knowing the difference between lockdown and non-lockdown notices is vital to managing personal liability. Read this so you can triage fast and keep options open.