How e-mails work in exchange server?

Hi!
We are experiencing a little problem with how the email works in our computers. We are currently running Windows 2003 SBS SP2 and we are hosting our own exchange server of version 6.5; build 7638.2 SP2.
The problem is there are times when an e-mail sends to a different account even after we have entered the correct intended receiver.
The incoming mail doesn’t show up in the log and we couldn’t track it using the message tracking Centre either.
We have already tried turning off all filters but the same thing happens. Please help, thank you.
