Checking the server logs to evaluate which emails were sent to <someaddress@domain,com> is the first action the analyst should do to evaluate the potential impact of this security incident. Server logs are records of events or activities that occur on a server, such as email transactions, web requests, or authentication attempts. Checking the server logs can help to determine how many emails were sent to <someaddress@domain,com>, when they were sent, who sent them, and what they contained. This can help to assess the scope and severity of the incident and plan further actions .

The calc entry is a suspicious registry entry that should be investigated first by the analyst. The calc entry is located in the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run registry key, which is a common location for malware to persist and execute on system startup. The calc entry has a data value of "C:\Windows\System32\calc.exe", which is the path to the legitimate Windows Calculator program. However, this program does not need to run on system startup, and it could be a disguise for a malicious executable that has replaced or renamed the original calculator program. The calc entry could also be a sign of a fileless malware attack, where the attacker uses the legitimate calculator program to execute malicious commands or scripts in memory1.

Anti-tamper is a process that protects a system or device from unauthorized changes or modifications. It can also log and report any attempts to alter the system or device. The company is using anti-tamper to ensure the appliance is not altered from its original configured state. CI/CD, software assurance, and change management are not processes that specifically deal with unauthorized changes. Reference: https://www.acq.osd.mil/se/briefs/16943-DoD-AT-Overview-Brief.pdf

https://legalshred.com/degaussing-vs-hard-drive-shredding/
The best and most secure method of rendering hard drive information completely unusable is to completely destroy it through hard drive shredding Shredding is a method of physically destroying storage media files by cutting them into small pieces using a machine called a shredder. Shredding can ensure that confidential data from storage media files is sanitized so the drives cannot be reused, as it makes it impossible to recover any data from the shredded pieces.

DKIM, or DomainKeys Identified Mail, is an email authentication method that uses a digital signature to let the receiver of an email know that the message was sent and authorized by the owner of a domain1 DKIM helps prevent phishing emails that spoof or impersonate other domains by verifying the identity and integrity of the sender. DKIM works by adding a DKIM signature header to each outgoing email message, which contains a hash value of selected parts of the message and the domain name of the sender. The sender's domain also publishes a public key in its DNS records, which can be used by the receiver to decrypt the DKIM signature and compare it with its own hash value of the message. If they match, it means that the message was not altered in transit and that it came from the claimed domain.