I’m describing it as a “Hacker’s Replacement for GMail” as opposed to just a “Replacement for GMail” because it involves a good deal of familiarity with Unix (or at least, to set up and debug the whole system it did. Perhaps following along is easier). But, the end result is powerful enough that for me, it is worth it. I finally switched over to using it primarily recently, confirming that all works as expected. I wanted to share the instructions in case they prove useful to someone else setting up a similar system.