Nothing is simple when you have a million users. Sure, it's quite easy to do a quick search and delete on a small number of mailboxes but for a million? It's a shit ton of servers and mailboxes to search through. At least it works better than it used to... Still, i'm surprised that they didn't restrict who could send to the distribution group (I"m pretty sure they are using MS Exchange.)
There are lots of examples for scripts to do the search & delete but it probably has to be broken up into lots of smaller batches to avoid killing servers or hitting memory limits.
Get-Mailbox -OrganizationalUnit "domain.local/Users" -resultsize unlimited | Search-Mailbox -SearchQuery 'Subject:"PDF version of memo" ' , 'From:"email@example.com” ',”Sent:03/02/2015" –DeleteContent -TargetMailbox “firstname.lastname@example.org” -TargetFolder “Delete” -loglevel full