not receiving eMail on yahoo from a specific address


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Hello everyone, I am at my wit's end about an issue with yahoo mail and am hoping to get some help/input on here.
I have two yahoo eMail addresses, a personal one and one that I am using for everything related to the volunteer work I do for a charity.
The same applies for another volunteer.
If we need to discuss charity matters, we both use our "charity" addresses for communication. We have done so for about three years now, sometimes daily, but at least a few times a week.
About a week ago, we have stopped receiving eMails from each other's charity addresses. It is just between these two, and both ways. We have tried other ways and they all work. Personal one to charity one both ways work. We both receive eMails from other parties to our charity eMail addresses. But just the two charity addresses are not talking to each other any more.
We both have extensively double checked all suggested errors that might accidentally have crept in. Checked filters, made sure we haven't blocked each other, we have both whitelisted each other anyway, checked spam folder etc.
We are pretty much certain that this is not an operator error, neither on mine nor the other volunteer's side. But I refuse to subscribe to yahoo's Plus service to get in touch with them.
Is there anything that's left for us to try, or are the only options to either accept that we can't communicate directly through our two charity eMail addresses and have to use a workaround for the time being, or to pay to get in touch with yahoo?
If anyone has any ideas on what we can check or try, please do let me know!
Thank you