Tray messages appearing when not expected

Brian's Mail list account <bglists@...>

I just downloaded a recent master snap, and every so often, while editing an emial or reading one I'll suddenly here Dropbox up to date. Anyone know why?
Is this some new feature I'm unaware of?

Sent via blueyonder.
Please address personal email to:-
briang1@..., putting 'Brian Gaff'
in the display name field.

Join to automatically receive all group messages.