We should scan emails using AntiVirus signatures to make sure there are no viruses attached in emails.
Note: A virus signature is the fingerprint of a virus. It is a set of unique data, or bits of code, that allow it to be identified. Antivirus software uses a virus signature to find a virus in a computer file system, allowing to detect, quarantine, and remove the virus.
SenderBase is an email reputation service designed to help email administrators research senders, identify legitimate sources of email, and block spammers. When the Cisco ESA receives messages from known or highly reputable senders, it delivers them directly to the end user without any content scanning. However, when the Cisco ESA receives email messages from unknown or less reputable senders, it performs antispam and antivirus scanning.
Reference:
/b_ESA_Admin_Guide_12_0/b_ESA_Admin_Guide_12_0_chapter_0100100.html

Explanation/Reference: https://www.cisco.com/c/en/us/solutions/security/secure-data-center-solution/index.html#~products

The various suspicious patterns for which the Cisco Tetration platform looks in the current release are:
+ Shell code execution: Looks for the patterns used by shell code.
+ Privilege escalation: Watches for privilege changes from a lower privilege to a higher privilege in the process lineage tree.
+ Side channel attacks: Cisco Tetration platform watches for cache-timing attacks and page table fault bursts.
Using these, it can detect Meltdown, Spectre, and other cache-timing attacks.
+ Raw socket creation: Creation of a raw socket by a nonstandard process (for example, ping).
+ User login suspicious behavior: Cisco Tetration platform watches user login failures and user login methods.
+ Interesting file access: Cisco Tetration platform can be armed to look at sensitive files.
+ File access from a different user: Cisco Tetration platform learns the normal behavior of which file is accessed by which user.
+ Unseen command: Cisco Tetration platform learns the behavior and set of commands as well as the lineage of each command over time. Any new command or command with a different lineage triggers the interest of the Tetration Analytics platform.