Drop support for today@trailmix.life

We are now sending/receiving emails using unique reply emails. For the next couple days, we are still processing emails that arrive at `today@trailmix.life` just in case folks reply to older emails in their inbox.

Once we stop getting emails at `today@trailmix.life`, we can merge this PR to stop supporting it.
I usually avoid memoizing like this unless I've seen that it's necessary for perf reasons, but I don't feel super strongly that this needs to be changed.