I just got this email today, about one of my old certificates:
This is a known issue - see here
Although it does say
Update: As of today’s Boulder release6 we have shipped fixes to each of the linked issues here. If you receive a renewal email that you think you shouldn’t have, check the date. If it’s after March 17, 2016 6:32PM UTC, start a thread describing the details and we’ll take a look. Thanks for your patience!