I will echo many others here: It’s going to be rough getting good deliveries. While you are planning on running a proxy, that is basically the same as running an open port where your server is. While it may seem to be a good idea to send email from a random AWS address, it really isn’t. Unless you are behind an IP that is specifically trusted as an email source, your traffic has a higher probability of getting dropped. (Many dynamic IP ranges for home internet connections are marked as invalid or untrusted sources, btw.)
Additionally, email servers are a hot commodity, especially if they are not blocked (yet) by the larger filter providers. All it takes is one or two reports or a poorly configured firewall/IDS to auto-trigger a submission of your IP address as “bad”. By hot commodity, I mean you are going to get fuck tons of vulnerability scans. It’s not the end of the world, but it’s super annoying.
If I was operating as a Jr. Security Analyst again and saw and sus traffic coming from your address, I would submit a block and not think twice about it. Hell, most of those types of blocks are automated anyway.
However, if you do set one up and all is golden, great! It’s worth the experience but something I won’t ever do again. (Yes I did run my own email server before.)
Old hardware is awesome to reuse most of the time but it’s not nearly as efficient as our hardware today.
It’s probably good to just properly recycle the old gear and spend $200 on a mini-PC from Amazon that has three times the power all while using less electricity.
I usually completely tear down old equipment into is raw materials, the best I can. It’s less likely to be shipped off to another country for uncontrolled destruction and I get more money back for the materials.