Lost some sent emails or emails never reach the recipients

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Case Study - Lost some sent emails or emails never reach to the recipient

Situation: there are two similar cases.

Case 1: the client used to have one public IP point to OWA server and another IP for regular mail. For example x.x.x.195 points to mail.chicagotech.net amd x.x.x.196 points owa.chicagotech.net. When they upgraded exchange 2000 to 2003 with a front-end/back-end topology, both server point to one IP.

Case 2: the client just changes a public IP point to a new Exchange server.

Problem: Some recipients called to the senders and told them they hadn't received any email the senders sent. The senders never received NDR.

Troubleshooting: 1. Used telnet test destination ad found the recipient's server blocking the client's mails.

2. Called the recipient's administrator and we were told that that the mail was coming but from owa.chicagotech.net instead of mail.chicagotech.net and. Therefore the mail was not resolving to the organization and they considered owa.chicagotech.net as a spammer.

Conclusion: This behavior occurs if the destination server cannot resolve the domain that you are using as your return address. For example, if your server's domain is nonexist.chicagotech.net and you are trying to deliver a message to a domain called ms-mvps.com and this ms-mvps.com has Reverse Lookup enabled on its SMTP server, mail delivery will fail. This occurs because the ms-mvps.com will perform a reverse lookup on nonexist.chicagotech.net and then it will consider nonexist.chicagotech.net as a spammer.