The Minecast documentation provides this for the error you are receiving:
Quote:
Email rejected due to security policies (E.g. MCSpamSignature.x.x) A signature was detected that could either be a virus, or a spam score over the maximum threshold. The spam score isn't available in the Administration Console.
If you aren't a Mimecast customer but have emails rejected with this error code, contact the recipient to adjust their configuration and permit your address. If unsuccessful, your IT department can submit a request to review these email rejections via our Sender Feedback form.
Anti-virus checks cannot be bypassed. Contact the sender to see if they can stop these messages from being blocked. Anti-spam checks can be bypassed using a Configuring Permitted Senders or Auto Allow policy. Rejected emails can be viewed in your Outbound Activity and searching for the required email address.
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As a first step, I would suggest sharing this information with your recipient. If unable to resolve the issue in this fashion, I would then use the Sender Feedback form, and see what Minecast have to say about the issue.