mimecast
Mimecast provides “cloud cybersecurity services for email, data and web,” specializing in cloud-based email management for Microsoft Exchange and Microsoft Office 365, including security, archiving, and continuity services to protect business mail. To clients and users, Mimecast may seem like an integrated Microsoft add-on, but from a sender’s perspective, Mimecast is effectively its own mailbox provider. Typical Mimecast customers point their domains’ MX records at Mimecast infrastructure and any inbound mail is rejected or accepted by that infrastructure before being logged, processed, archived (if archiving is enabled), before being forwarded to the actual end user mailbox for the email message’s destination. Mimecast hosts inbound email services for many thousands of email domains. When cross-referenced against the top 10 million domains, Mimecast shows up in the MX record for over 37,000 of those domains. While this is smaller than Google (~800,000) and Microsoft (~727,000), a lot of the entities you’ll see