Thanks Matt. After waiting for several days with no answer, and not wanting to burn one of my precious few support tickets on a single domain issue, I started looking around for what could be causing the issue. I found this setting was unique in the settings.json file for the affected domain:
I noticed all the other domains on the server (91 of them) all had this set to 1. I changed that setting for this domain, restored the smartermail service and everything started working properly for the previously affected domain. I haven't noticed any other issues since making this change yesterday.
Is there something else that I need to check, or could it be as simple as this not getting set properly during the upgrade? I'm a little nervous that other issues are lurking for this (and potentially other) domain, but I don't have any concrete evidence to support that concern.
Any insight, thoughts or things to check would be greatly appreciated.