I have an Exchange Server 2003 running on Windows SBS 2003. As of about a week ago we do not receive some of our email. Some emails get to us while other do not. The ones that do not reach us are consistent from the same senders. I did a message tracking and it looks that those messages do not even reach our server. Our ISP keeps our DNS server, there are correct A, MX and PTR records for our server. The last message logged on my server from one of these senders is on Thursday last week. The only thing I changed on Thursday was our Security Certificate.