Exchange: NDR 571 - MAIL REFUSED - Reverse DNS failed; cannot resolve the domain in the HELO command

Summary:

You receive the following NDR when sending to a third party domain:


Your message has encountered delivery problems
to the following recipient(s):

user@externaldomain.com
Delivery failed
571 - MAIL REFUSED - Reverse DNS failed; cannot resolve the (yourmailserverhostname.domain.com) domain in the HELO command.

You verify that your domain does indeed have a Reverse DNS Record.

Cause:

The third party domain is performing HELO lookups, also known as forward DNS lookup. When initiating a SMTP session with a third party domain, the domain validates your domain name by performing a forward check, Domain to IP rather than IP to Domain in reverse DNS lookups. If you designed your DNS namespace for Active Directory and chose a namespace different than your public domain name ie. company.local, your Exchange server by default will advertise as HELO I'm hostname.company.local. The third party domain will attempt to lookup company.local in DNS and will fail and thus reject your E-Mail.

Resolution:

1. Open Exchagne System Manager. Navigate to Administrative Groups, Site, Servers, Servername, Protocols, SMTP, Default SMTP Virtual Server,

2. Highlight Default SMTP Virtual Server, right click properties.

3. Go to the Delivery Tab, and click Advanced Button.

4. In the "Fully-qualified domain name: field, enter your fully qualified external domain name. Click Check DNS to verify that it resolves.

5. Restart SMTP service.

James Chong
MCSE M+, S+, MCTS, Security+
msexchangetips.blogspot.com


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