@robert I imagine you are fully aware, but for those out of the loop, a standard feature of Discourse is that when you get an email notification for a DM or thread you are tracking, you can reply directly to the email and it will perform the appropriate action on the site through your account.
It looks like this when you receive an email for a DM:
It looks like this when you receive an email for a tracked thread:
While organizing the Pizzeria Sei event, a few people mentioned that they had tried to respond to my DMs via email, but days later received undelivered/bounce-back emails. It appears that something is off with the configuration of this site and/or its email servers. Just wanted to let you know, either so that you can try and resolve it, or perhaps turn off this feature entirely if fixing it seems infeasible.