The issue where a customer continues to receive emails despite unsubscribing and being deleted from the data extension likely arises because the email address in All Subscribers still exists and is active. In Marketing Cloud, the All Subscribers list acts as a master list overriding subscription statuses in individual data extensions. Thus, even if a customer is removed from a specific data extension, if their status in All Subscribers is not updated to 'Unsubscribed,' they will continue to receive communications.
Reference: Salesforce Help - All Subscribers List

For a large retail company migrating to Marketing Cloud, proper IP warming is critical:
A). IP ramp-up takes four to six weeks to be able to fully send to all 3 million customers: It's essential to gradually increase the volume of emails sent from a new IP address to build a positive reputation with Internet Service Providers (ISPs).
D). IP ramp-up is important to establish a positive sender reputation: Establishing a positive sender reputation with ISPs during the IP warming process helps ensure high deliverability and avoids being flagged as spam.
These responses highlight the necessity and methodology of IP warming when transitioning to a new email platform to maintain deliverability standards.
Reference: Salesforce Help - IP Warming

Before using Goals in Journey Builder, consider the following:
B). Goals are created to evaluate journey performance: Goals help measure how well a journey is performing against specific objectives, allowing for adjustments and improvements.
C). Goals can act as exit criteria: When a contact meets the goal criteria, they can be set to exit the journey, making goals a powerful tool to dynamically manage contact flow based on behavior or achievements.
D). Contacts are evaluated against the goal after a wait activity: This setup ensures that contacts have time to perform the expected action before being evaluated against the goal, which can be crucial for accurately measuring engagement and responses.
Reference: Salesforce Help - Journey Builder Goals

Segment by domain to limit number of sends to each email provider is the option for segmentation that aligns with the IP Warming process. IP Warming is the process of gradually increasing email volume from a new or unused IP address to build trust and credibility with Internet Service Providers (ISPs). Segmenting by domain means dividing the email list into smaller groups based on the email domain, such as gmail.com or yahoo.
com. By segmenting by domain, marketers can limit the number of sends to each email provider and avoid being flagged as spam or blocked by the ISPs.