I am installing a process that reads emails from external clients. Some clients

That's correct, only CC and TO are supported for routing alias, this seems to be related to the nature and meaning of BCC who are those recipients are those being discreetly informed of the communication and cannot be seen by any of the other addressees.

You can check with your MS Exchange Administrator if he can create a rule to forward messages where the alias is in the BCC to the alias itself (and delete the original message) so that the alias email address is moved to the TO in the new email.

Looking at the options I have in Outlook I don't think it's possible to inspect the BCC but you may want to double check with them.

The reference number for this feature to support BCC is AN-46447.

-Eduardo Fuentes
Appian Technical Support