The new Exchange Server component, aptly named Microsoft Exchange Emergency Mitigation (EM) service, builds on Microsoft's Exchange On-premises Mitigation Tool (EOMT) released in March to help customers minimize the attack surface exposed by the ProxyLogon bugs.
EM is an EOMT version built within Exchange Server that works with the cloud-based Office Config Service (OCS) to download and protect against high-risk bugs with known mitigations.
It runs as a Windows service on Exchange Mailbox servers and it will be automatically installed on servers with the Mailbox role after deploying the September 2021 (or later) CU on Exchange Server 2016 or Exchange Server 2019.
It is important to note, EM is not a replacement for Exchange Security Updates, which still need to be part of your Patch Management.
Sources:
New Microsoft Exchange service mitigates high-risk bugs automatically (bleepingcomputer.com)