Can you please respond to following query. If there are multiple email addresses in a Sendable DE, and one of them is Subscriber key, will the email be always send to the Subscriber Key (Email Address).
Your SubscriberKey should be of Text datatype and you should only have one field of EmailAddress datatype. When sending to a Sendable DE like that, the email will be sent to the address in the EmailAddress type field, if the subscriber has not been sent to from your account before. If the subscriber has previously been sent to, the value in the EmailAddress field is ignored and the email address from All Subscribers is used.
Triggered Sends operate slightly differently in that with the correct configuration, the email address from from your API call/Data Extension EmailAddress field is used to override All Subscribers for existing subscribers.