https://resources.sendinblue.com/en/category/faq/8transactionnel-faq/SendinBlue is our service for sending newsletters (or emails targeted to many contacts at once), and SendinBlue SMTP is for sending non-promotional emails (or custom emails for a single contact).
What is a transactional email? A transactional email is sent as a result of a contact’s request or action. For example: a purchase confirmation, a reset password email, alerts, etc.
Synergies between SendinBlue and SendinBlue SMTP are essentially technical :
improve your deliverability
hard-bounce pooling and avoid returning an email to a contact that has resulted in a hard-bounce. With SendinBlue SMTP, your hard-bounces will automatically be imported to your SendinBlue account to avoid making the same mistakes with your regular newsletters.