Mimecast emails will always come from [email protected]
If you click on Release, the message will be removed from the hold queue and will be delivered to your Inbox ( provided it passes virus and attachment scanning). Future messages from the sender are still subject to spam check, virus scanning and attachment scanning.
If you click on Block, the message will be removed from the hold queue and will not be delivered to your Inbox. The sender's email address is added to your personal block list. Future messages from them are rejected and won't be delivered to your Inbox.
If you click on Permit, the message will be removed from the hold queue and will be delivered to your Inbox (provided it passes other email security policies such as virus and attachment scanning). Future messages will bypass the spam checks and are delivered to your Inbox, provided they pass virus and attachment scanning