You receive an unencrypted e-mail from a colleague who wants to send you confidential information. How easy do you think it is to intercept this e-mail?
Internal e-mails can not be viewed or read by unauthorized persons.
Anyone who has obtained access to the network by appropriate means can read the e-mail.
E-mails are always sent from sender to receiver. Therefore no one else can gain access to them.
It is possible that the person sending the message is not the person refered to in the sender information.