For intra-corporate mail, email works just great, because the identity problem is solved. Trusted communications between 3rd parties just isn't a natural mail use case.

Not really. Hence the rise of intra-corporate chat software.

People instead prefer to join focused, well defined groups to send and receive intra-corporate communication.

Not bee CC'ed spammed all to h*ll about every little office detail that has zero impact on you and your job.