How to set the default "From" value for emails.

Hello Community, 

I have a client who has several shared mailboxes synchronized. Currently, two of them are parameterized for sending emails:

- One is used for customer communications 

- The other is used for communications with the partner

Whenever an e-mail is created from a case or an e-mail is replied to from within the case, "From" takes over one of these mailboxes by default. For example, when replying to an email from the partner, we want the "From" to be with the mailbox synchronized for communications with the partner, but it always assumes the other one for communications with the customer and we have to change it manually. 

Is there any way of dynamically changing the "From"?

Thank you.