If you want to avoid having the SMTP server create a new email list every time you send out emails, all you have to do is add the appropriate guide on the headers of the email.
What you are looking for is the following guide:
- mailing_list_id: your_mailing_list_id :
This allows all the recipients to be added using the same email list ID.
If you don't use the above guide on the header of the email, a new email list will be created based on the subject of the email you sent out.
Make sure to check the other guide which is available for you to use when sending out campaigns via our SMTP service.