How To – Mimecast
Block external email sender
A. Block via Email Users & Groups
2. Click on Administration Console
3. On left side select “Users & Groups”
4. Select “Profile Groups”
5. On Edit Group Root select Black List then click on Build > click on Add Email Addresses.
6. Type or paste the email address in Group Additions box and Add a note if you want.
B. Block via Email Delivery Menu
2. Click on Administration Console
3. On left side select “Email Delivery”
4. Select Managed Senders.
5. To add new email select Add > Add Managed Senders
6. Fill in the Managed Senders Selection details such as Sender Address, To Address and select BLOCK.
7. Try to search the sender email and it should appear as untrusted sender. this method only block sending email to specific recipients not block the email entirely.