If you want to avoid having the SMTP server create a new email list every time you send out emails, you must add the appropriate GUID on the headers of the email.
Look for the following GUID: mailing_list_id: your-mailing-list-id-here
This allows all the recipients to be added using the same email list ID.
If you don't use the above GUID on the header of the email, a new email list is always created based on the subject of the email you sent out.
Make sure to check the other GUID available for you to use when sending out campaigns via our SMTP service.