Some helpful guides provided by Mimecast.

If using Mimecast, admins may need to configure their Mimecast instance to accommodate Phin IPs, URLs, and domains.

Mimecast offers the following tools for configuration. If you have any issues with Mimecast, please reach out to the Mimecast support team.

Anti-Spoofing Policy to Allow Spoofing (Bypass)
Permitted Senders Policy
Preventing Mimecast from Re-Writing Phishing Links
Greylisting Policies