There doesn't appear to be a way to do this in Gmail and this only started happening recently.
Edit: This may do the trick:
https://downtimemonkey.com/blog/how-to-whitelist-an-email-address-in-gmail.php
I won't know until I receive more emails from IH8MUD as it doesn't unflag old emails.
For the last few weeks, email messages from IH8MUD to alert me to updates in subscribed threads have been getting flagged with the following header:
The ? in a circle goes to this page that theoretically gives some tips on how to resolve the issue...