Hello Mickey. Port 25 are closed by default on all server types, but it can be opened by request on all G3 plans: https://www.99stack.com/products/generation-3. Additionally, there is no block on port 587 so from any instance you'll be able to send emails via any third party forwarding service.
Submit your request to support department: https://www.99stack.com/company/contact as a ticket or by email. The request may include one or more ip addresses, for the server instances you wish to whitelist. This will be a dedicated and clean ip address for emailing.
Follow up questions may be required if you submit many requests in a short period of time. Keep in mind that ip "rotation", where you deploy a bulk of servers, approve them for mailing, and then throw them away after a few hours, only to immediately deploy a new range of servers and try to whitelist them as well, is not permitted. This tactic is commonly used by spammers and may get entire ip ranges blacklisted.