Block malicious emails

You should probably PREFER TO USE Mimecast INSTEAD OF THIS PROCESS

Block based on the sender's domain

Visit exchange rules

Then edit the Block email based on sender's domain like this:

Don't forget to add in the comments who asked about this and when
To access the comments click on more actions

In the end don't forget to press SAVE

Block based on the sender's email addresses

Go to the exchange rules and then:

Then edit the rule Block based on the sender's email addresses like this:

Don't forget to add in the comments who asked about this and when