Try this: https://support.99stack.com/books/tutorials/page/smtp-enabled-server-instances
Basically, you need to select a G3 region (A region with a data center id of 3xx), then flip the "Enable SMTP" switch during creation, and last but not least check up-front payment section and be sure to select at least 3 days. This will reserve your server and IP for 3 days which prevents IP rotation.
Once you've done testing I suggest you deploy a new server with at least 3 months of up-front payment as this will give you a over 74% discount.
