@xconde it absolutely _does not_ stay in your brain. it would take me a while to get back up to speed if I had to do it again and thank fuck it's not the kind of thing i have to do anymore.
we had three different transport layers all with very different and variable costs and speeds and a hostile admin on the far side of the most important uplink. it was nightmare adventures _every day_ and I do _not_ miss it.
but that was a sendmail.cf that both made it work and made the beardiest of neckbeards cry, and yeah. still proud of that part.