CRM Mail-Sharing issue | Public sharing vs. defined sharing roles

CRM Mail-Sharing issue | Public sharing vs. defined sharing roles

Hi,

While the issues surrounding mail-sharing rules have been discussed in this forum before, I couldn't quite find an answer to my specific problem and was hoping for input (and ideally a solution).

For the sake of making my argument, lets use the following setup:

Employee A-01
Employee A-02
->These two belong to branch A of the company and should always see all their email communications with their customers, but should not see any mails that employees of branch B send to customers

Employee B-01
Employee B-02
->These two belong to branch B of the company and should always see all their email communications with their customers, but should not see any mails that employees of branch A send to customers

All four employees work within the same Zoho CRM instance but are seperated through user roles.

What I would like: Whenever I add a new employee as a Zoho user to branch A or B, I want them to automatically share their emails with all other members of their respective branch. I assumed this would be possible by setting the sharing rules to "public" and defining the target roles like this:



However, what seems to happen is: As soon as the "public" sharing is enabled, all role/user restrictions are ignored an mails are shared across all roles. So now employees of branch A and branch B see each others emails, and this is a big problem for us. A few additional points:

->Excluding domains is not an option, as both branches share the same domain
->"Custom" sharing is not an option either, as this requires setting a manual checkmark ("Share Emails") for each individual customer, which quickly becomes unfeasible since we're talking about thousands of customers per branch. I also don't understand, why this has to be an opt-in. If we could simply default this to an opt-out, where all customers initially have the sharing-checkbox ticket, all problems would be resolved.

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My question(s) in a nutshell: Am I misunderstanding how mail-sharing is supposed to work in Zoho-CRM? How can I achieve our desired setup within the current Zoho CRM restrictions? 

Thanks!