Mimecast is a tool designed to catch potentially malicious
emails before they hit your inbox. When an email is put on hold, the intended
recipient is sent an email from Mimecast saying that an email has been put on hold. From
here, you can either permit/reject/block the email/sender. Though Mimecast does
a good job of blocking only malicious/spam emails, sometimes safe ones get
caught in the filter.
Knowledge Base Articles