Mimecast is used by the firm to scan all incoming and outgoing mail for any spam or malicious indicators.  Emails flagged for spam are placed 'on hold' in Mimecast.

Every few hours the staff will get a summary email of any emails placed 'on hold'.

You can review the email from the On Hold Messages section of the Mimecast Outlook plugin.

From there you can view any messages that are being held for spam, and choose to release, reject, or report the message.