I have a data extension in SFMC that is being populated with opt-ins from an external source. I would like to trigger an instant welcome email ONLY to anyone who is added to this data extension. How do I go about doing this? I want make sure I don't resend to the anyone who was already on this data extension and has received the email.
You would have to either do a Triggered email or an API event for a journey. Either way, you would have to use APIs to trigger them.
After that, you could use a DE as an exclusion list in the email deployment so you don't send the email twice.
If you are doing it as a journey then just select that people can only enter the journey once.