For this reason Project Athena has developed a system called Kerberos. Kerberos is able to verify that you are who you say you are and not impersonating someone else. Zephyr uses the Kerberos system to authenticate its users. If you and the sender passes authentication, then the message is marked with Authentic. This does not necessarily mean that something bad is occurring.
Any legitimate user can gain a new set of tickets by running the 'kinit' program.
In an extreme case it can be interpreted as someone tampered with the Kerberos ticket but a common cause is that you renewed kerberos tickets with kinit and thus these tickets do not concur with that which Zephyr is expecting. The next time you need to use kinit, then remember to repeat the above command.