Wrong delivery of messages in Exchange 5

Hello,

I am facing a rather strange situation with my exchange server 5.5 SP3.

I have a some users complaining that they sent email messages to other users and the mails are received from users that do not appear in the TO: section of the email message.

For examples:
I send a message TO: user1@somewhere.com and the message is reveived from user2@elsewhere.com. Note, that user1@somewhere.com received the message, too.

Do you know what is the problem?

How the mail message delivery is working?

What can cause the mail server to send the mail message to a user that is not appearing in the TO: section of the mail message?

PS: Except BCC…

Thanks in advance.