I'd advise to do the following:

- Block all three subjects on your transport servers

- Block SCR files as attachment

- Update your file level AV

- Update your Exchange AV

- Run manual scan for your Exchange AV to harvest mailboxes that received the virus already before def update

There might be more variants with more subjects later on, so the key is blocking SCR files, user awareness and make sure that latest AV defs are deployed.