I learned today that if you do not have the Unity_<servername> account in a particular OU to which is not running two-way replication, then Unity will not work in mix-mode. If you have not "assigned" a two-way CA within the OU to which the Unity_<servername> is located, then there is no possible way for Unity to log into Mailboxes that are located on an E5.5 server via mix-mode.
If you are working on the assumption that your environment has a two-way CA configured, ensure that this is the case to which your Unity accounts are located.
In my case.. all my Unity accounts were located in an OU that only had a one-way CA, and this is why my Unity server was unable to open Mailboxes in E5.5 via mix-mode.
None of the Cisco documentations "tells" you to verify this configuration, it all assumes that your Unity accounts are located correctly and that you know what you're doing if using mix-mode. In most cases, nobody assigns a whole container for two-way CA, and the default location where Unity adds the new accounts related to the install go into the defaul OU location. So, you're going to be required to move these accounts into a two-way CA granted OU.
Thanks to the TAC for making me verify that my Unity_<servername> account was replicating back to E5.5