This is from 2023, but this might still be a good read on how to get to a very low watt idle build, there’s a few more post on other older builds on that blog as well: https://mattgadient.com/7-watts-idle-on-intel-12th-13th-gen-the-foundation-for-building-a-low-power-server-nas/

The most important thing, if you intend to do outgoing email, is to get a reputable hosting service, where you’d have few/none bad neighbours. Unfortunately that often doesn’t come together with cheap, as cheap solutions often attract a certain clientele. I’ve also had more luck with slightly smaller (but still reputable) providers, i.e stay away from OVH / Hetzner and the like. If you want something cheap, it’d probably not be a good experience to self-host.

With my self hosted solution I have no concerns about deliverability, and only very few occasions had blacklisting issues (due to neighbours in the /16 range).