It's relatively simple once you know how it's done 🙂
First thing you need is a G3 server instance, that's any plan deployed in any 3xx region, for instance 310 for Frankfurt, Germany or 304 for Dallas, Texas. Chose your plan and operating system in the drop down lists as you would normally do for any deployment.
Now to the next step, after selecting a G3 region, you can now click on "Enable SMTP", this will ensure that your server is not limited by any firewall restrictions and leave port 25, 465 and 587 open.
Proceed to the final step, notice that nobody is allowed to buy a on demand server with SMTP enabled, this because it was abused by spammers who managed to blacklist entire ip ranges and ruin the experience for everyone. The solution is to click on the "Up-front" tab, which will allow you to pay in advance for a server instance reserved for a longer period of time.
Now, anything longer than 3 days will allow you to enable SMTP, the longer time you reserve a server instance for, the more discount you get, up to 20%. But 3 days is absolute minimum, starting at $0.038/hr or $2.8 in total.
This is non refundable, and although a bit expensive for testing purpose, it's enough to financially ruin spammers. Some say the spam networks will never financially recover from this, but it's good for you since you can be sure you'll get a clean ip address with good reputation 😉
Also note that if you reserve for 3 months or longer it's only 2/3 of the price you pay for one month. A huge discount that is if you intend to play nice and respect the anti-spam laws.