Multiple Mailing "Default Sources"

Hi Everyone, 

I’m trying to have more than one mailing “Default sources” under the Mailing and Printing Tab of Purchase Order Preferences, as seen in below screenshot.
 

The issue is that, when i create a PO logged in to each of the branch, then when i click “Email PO”, only for the first branch’s PO does send the email successfully. For the rest of the 2 branches POs, clicking “Email PO” throws an error. Please see below screenshot for the specific error:
 

Please help!

Thanks,
​​​​​​​Wandile