We don't offer an SMTP infrastructure at this moment. However, you may find this short workaround useful:
1. Create a new automation.
2. Set the trigger as “When a subscriber joins a group”.
3. Add an email step.
4. Start adding recipients to this group using our API.
Each time a recipient is added, they will receive the email. You can find more details in our API documentation and on our automation page.
Alternatively our partner company, MailerSend, does offer SMTP relay. Try it out today when you sign up for free!