By using Tracking Extract, File Transfer, and Import activities from Automation Studio, you can extract the data from the subscribers who have had emails fail due to held status within the last 60 days [1][2], and then transfer the data into a data extension that can be used as an Entry Source for Journey Builder. This will allow you to create an SMS message that can be sent to the subscribers to ask them to update their email address.
Additionally, Automation Studio provides the ability to schedule and automate the process so that the data is regularly updated and the SMS message is sent on a regular basis. For further information, please refer to the following Salesforce documentation:
* Automation Studio Overview: https://help.salesforce.com/articleView?id=mc_as_overview.htm&type=5 * Automation Studio Activities: https://help.salesforce.com/articleView?id=mc_as_activities.htm&type=5 * Data Extensions Overview: https://help.salesforce.com/articleView?id=mc_es_data_extensions.htm&type=5 * Journey Builder Overview: https://help.salesforce.com/articleView?id=mc_jb_overview.htm&type=5

Supporting inbound messages in each country would require additional long code implementation for MobileConnect. A long code is a standard phone number that can send and receive SMS messages. Each country has its own regulations and requirements for long codes, so a single long code cannot be used for all markets. The other functionalities do not tie to additional long code implementation. References: https://help.
salesforce.com/s/articleView?id=sf.mc_moc_long_codes.htm&type=5

The Transactional Messaging API can be used to send email to customers who log in for the first time on a new IP. The Transactional Messaging API allows for fast and reliable delivery of transactional messages, such as password resets, order confirmations, or account alerts. The message can include information related to the login IP by using personalization strings or AMPscript variables. References: https://developer.
salesforce.com/docs/marketing/marketing-cloud/guide/transactional-messaging-api.html https://developer.
salesforce.com/docs/marketing/marketing-cloud/guide/transactional-messaging-personalization.html

A limitation that the consultant should consider when recommending Field-Level Encryption is that Field- Level Encryption doesn't support segmenting, filtering, or querying encrypted fields. Field-Level Encryption is a feature that allows marketers to encrypt sensitive data in Marketing Cloud using encryption keys and algorithms. However, encrypted fields cannot be used for segmenting subscribers using filters or queries, as they cannot be evaluated or compared with other values. References: https://help.salesforce.com/s
/articleView?id=sf.mc_overview_field_level_encryption.htm&type=5

The number of business units that NTO needs to purchase depends on several factors, such as the number of brands or geographies they operate, the need for data or content segregation between them, and the number of Marketing Cloud users they have or plan to have. These factors affect the complexity, scalability, and cost of their account hierarchy and governance. The number of Sales Cloud sandboxes or testing/development business units are not directly related to the number of business units they need to purchase, but rather to their testing and deployment strategy. References: https://help.salesforce.com/articleView?id=sf.
mc_overview_business_units.htm&type=5 https://help.salesforce.com/articleView?id=sf.
mc_co_best_practices_for_creating_a_business_unit_structure.htm&type=5