Messages disappearing to POA

Dear friends,
We have a 6.5 GW system running for more than two years now, running 6.5.7
version.
Now we started to have an interesting problem. I can follow the message
until it leaves the MTA, but the message don´t get to the user mailbox. Some
messages come addressed to 5 or 6 addresses but only 1 or 2 users don´t get
their message, the other users receive without trouble.
I don´t know where I can start troubleshoting this problem. Can somebody
help me?

Regards,
Marcos Vianna