Re: 5. Row, "Use for Notifications" activated,
OK, this is the reason.
If you had that off, then each account would get 1) its own notification icon and 2) its own sound.
But I understand that with 18 accounts, you don't want one icon per account
I'll file a bug and see what we can do.
What's happening is - the "smart folder (combined)" notification gets updated after each account finishes its mail check, and so the code doesn't really know which sound to use.
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It would help if you could also make a debug log with this problem and send it to support / aqua-mail / com. Please include a little "note" about "which account is which" (in the log).