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Hi, <br />
For testing purpose add the IP address and FQDN of exchange in host file and check mail flow. <br />
If this works fine then you need to check the settings in your internal DNS server <br />
Milind
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Hi,
For testing purpose add the IP address and FQDN of exchange in host file and check mail flow.
If this works fine then you need to check the settings in your internal DNS server
Milind

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#554 5.4.4 SMTPSEND.DNS.NonExistentDo<wbr />main; nonexistent domain ##
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#554 5.4.4 SMTPSEND.DNS.NonExistentDomain; nonexistent domain ##

How do I resolve the following on our exchange server when some end users in our org. try to reply back to certain recipients?

Interactions between email servers and clients are governed by email protocols. The three most common email protocols are POP, IMAP and MAPI. Most email software operates under one of these (and many products support more than one).  The correct protocol must be selected, and correctly configured, if you want your email account to work.

Email Protocols

Windows Server 2008 and Windows Server 2008 R2, based on the Microsoft Vista codebase, is the last 32-bit server operating system released by Microsoft. It has a number of versions, including including Foundation, Standard, Enterprise, Datacenter, Web, HPC Server, Itanium and Storage; new features included server core installation and Hyper-V.

Windows Server 2008

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.