The current state on mx2001 is puppet disabled, exim4 stopped and kernel has been downgraded (manually via grub boot menu AFAIK).
Additionally recent changes to MX priority and ldap routing have been reverted -- Mx1001 is currently the primary both in terms of MX and exim smarthost priorities.
AIUI we'll be sticking with the present 5.10.0-8-amd64 kernel for the holiday period, and postpone follow up kernel upgrades until the new year. So I think it'd be worthwhile to persist the kernel config, in case mx2001 is rebooted.
Creating this task to track steps to stabilize mx2001 and bring it back into service.