Canada's anti-spam legislation (CASL)1 at the time was more strict than the regulations in other countries (e.g. CAN-SPAM Act in the U.S.). Notably, it applied to all emails sent to people in Canada, including those sent from other countries (of course, likelihood of enforcement is another issue).
In general, a commercial electronic message can only be sent to users with their consent (express consent to receive messages, or implied consent through e.g. club membership or existing business relationships).
Additionally, receivers must be able to unsubscribe or opt out.
What part is mandatory?
It is not mandatory in the sense it is required by law.
But rather Microsoft considers that the communication is essential to its service provided to the user. If you want to opt out this kind of communication, Microsoft is signaling that it can/will end the customer relationship, that is, the only way user can opt out is to stop using the relevant service.
The term "mandatory service communication" is and was used in Microsoft's Privacy Statement and other relevant user agreements
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You can choose whether you wish to receive promotional communications from Microsoft by email, SMS, physical mail, and telephone. If you receive promotional email or SMS messages from us and would like to opt out, you can do so by following the directions in that message. You can also make choices about the receipt of promotional emails, telephone calls, and post by signing in with your personal Microsoft account, and viewing your communication permissions where you can update contact information, manage Microsoft-wide contact preferences, opt out of email subscriptions and choose whether to share your contact information with Microsoft partners. If you do not have a personal Microsoft account, you can manage your Microsoft email contact preferences by using this web form. These choices do not apply to mandatory service communications that are part of certain Microsoft products, programmes, activities or to surveys or other informational communications that have their own unsubscribe method.
CASL does not require the sender to signal if it's necessary to its contract or not, but it does require the sender to identify the opting-out mechanism. In this case, the statement informs the user that this particular communication is integral to its service and cannot be opt out separately from the service.
Unlike the later European GDPR where a necessity test is used, CASL does not have this requirement and the contractual parties can define if a communication is integral as they want by means of express consent via user agreements (subject to other federal and provincial data protection rules).
- Or its really long long title: An Act to promote the efficiency and adaptability of the Canadian economy by regulating certain activities that discourage reliance on electronic means of carrying out commercial activities, and to amend the Canadian Radio-television and Telecommunications Commission Act, the Competition Act, the Personal Information Protection and Electronic Documents Act and the Telecommunications Act