Thanks for your latest clarification on the current Yahoo problems.
Since the delivery lag, or even entire absence of Yahoo email service, seems to be “user, or perhaps ISP specific”, then Yahoo must be having some difficulty with exchanging emails from certain ISP routing addresses, or regional network trunk lines.
Glad to hear that you found that Yahoo is working on their now “known”, (non-Group specific), problem, and may hopefully resolve it soon.
This may explain why only some random users are still having such difficulty. That major Yahoo outage on the weekend might indicate the severity of “their problem”, so they had to completely shut down service to isolate the cause, a bit better, perhaps a server virus flush at the same time.