I think I've seen this at least once before, many many moons ago, and indeed, I got one today as well (confirming that they have confirmed an issue I reported). My guess is someone checked to see if we were online, and because we were not, they just left these messages instead (I was online on PlayOnline, but not in game). A thread ID would have been a nice addition, but perhaps they didn't have those handy. :]
More than a few times within the past year or so I've been contacted in-game due to my bug reports (asking confirmation on if I can still re-produce the issue and such).
Regarding '1,11', perhaps something you reported was (or should have been) fixed in the January 11/12 version update?
The PlayOnline e-mail service was discontinued back in 2019, and these messages show up in a dedicated window up on top of everything in the PlayOnline viewer. The only option is to close the message window (the other button is 'proceed' which is unavailable):
While it does get saved as a PlayOnline message, it can't be replied to.
Actually... I guess they only show up as messages (not as a window, too) if one isn't online on PlayOnline at the time.