Engaging stakeholders is the most important step when developing an information security strategy, as it ensures that the strategy is aligned with the business objectives, risks, and needs of the organization.
Stakeholders include senior management, business units, IT staff, customers, regulators, and other relevant parties who have an interest or influence on the information security of the organization. By engaging stakeholders, the information security manager can gain their support, input, feedback, and buy-in for the strategy, as well as identify and prioritize the security requirements, expectations, and challenges.
References = CISM Review Manual, 27th Edition, Chapter 4, Section 4.1.1, page 2131; CISM Online Review Course, Module 4, Lesson 1, Topic 1

The first step when integrating information security into HR management processes is to assess the business objectives of the processes, which means understanding the purpose, scope, and expected outcomes of the HR functions and activities, and how they relate to the organization's strategy and goals. The assessment will help to identify the information security requirements, risks, and controls that are relevant and applicable to the HR processes, and to align the information security objectives with the business objectives.
References = CISM Review Manual 15th Edition, CISM: Overview of domains [updated 2022]

Developing a business case to replace the system is the FIRST course of action that the information security manager should take, because it helps to justify the need for a new and effective email filtering system that can prevent or reduce phishing incidents. A business case should include the problem statement, the proposed solution, the costs and benefits, the risks and assumptions, and the expected outcomes and metrics.
References =
CISM Review Manual, 16th Edition, ISACA, 2020, p. 42: "A business case is a document that provides the rationale and justification for an information security investment. It should include the problem statement, the proposed solution, the costs and benefits, the risks and assumptions, and the expected outcomes and metrics." Email Filtering Explained: What Is It and How Does It Work: "Email filtering is a process used to sort emails and identify unwanted messages such as spam, malware, and phishing attempts. The goal is to ensure that they don't reach the recipient's primary inbox. It is an essential security measure that helps protect users from unwanted or malicious messages." Cloud-based email phishing attack using machine and deep learning ...: "This attack is used to attack your email account and hack sensitive data easily."

Explanation
Threat analysis is a process that is used to identify and assess the external influences or threats that could potentially affect an organization's information security. It is used to identify potential risks and develop strategies to mitigate or reduce those risks. Threat analysis involves analyzing the environment, identifying potential threats and their potential impacts, and then evaluating the organization's current security measures and developing strategies to address any deficiencies.