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:
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