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Exchange Server 2016 Cumulative Update 1 May Cause Edge Transport to Reject Email to Valid Recipients

News of a bug with Exchange Server 2016 Cumulative Update 1 has emerged, with some customers running Exchange 2016 Edge Transport servers finding that the Edge server rejects emails sent to valid email addresses.

Details of the bug have been added to the release notes for Exchange 2016.

Edge Transport servers can reject mail sent to valid recipients Exchange 2016 Edge Transport servers may reject messages sent to valid internal recipients when the following are true:

  • Exchange 2016 Cumulative Update 1 (CU1) is installed on the server.
  • Recipient validation is enabled on the server.

When an Edge Transport rejects a message because of this issue, the sender will receive a non-delivery report (NDR) with the status code 5.1.10, and the error Recipient not found by SMTP address lookup. The recipient won’t receive the message.

From testing it appears that this bug impacts Edge Transport servers receiving email directly from the internet. Edge Transport servers that are used for hybrid mail flow with Exchange Online do not appear to be affected by this bug.

Microsoft’s advice to customers affected by this bug is to either:

  1. Disable recipient validation on Exchange 2016 CU1 Edge Transport servers
  2. Route inbound email to an Edge Transport server that has not been upgraded to Exchange 2016 CU1
  3. Route inbound email directly to a Mailbox server

If you prefer not to bypass their Edge Transport server, and don’t have a non-CU1 Edge to route email to, then disabling recipient filtering on the Edge Transport server involves running the following command on the Edge Transport server:

Paul is a Microsoft MVP for Office Servers and Services. He works as a consultant, writer, and trainer specializing in Office 365 and Exchange Server. Paul is a co-author of Office 365 for IT Pros and several other books, and is also a Pluralsight author.
Category: Exchange Server


  1. LT says:

    Seems like a pretty major bug to slip through the Microsoft QA process. Recipient filtering is a critical part of many admin’s security protocol. Hope they patch this pronto.

  2. Samim Shaikh says:

    Bugs for internal roles can be understandable….. Border gateway role is having bugs. I would say not a good stage or time to migrate…

  3. Sonny sonikar says:

    If you have Exchange 2010 on-premises and want to install Ex2016 edge server, we could not find Exchange management console for it (when we installed on dev servers). Anyone else experience this ? Or have work around ?

  4. BW says:

    I run Exchange 2016 Edge role and I have just started to use IPv6 (statically assigned addresses) on my Exchange servers but when I run:

    netstat -aon | find “:25 ”

    it shows that port 25 is only listening on an IPv4 address:


    If I look in the email headers I can see it using IPv4 when connecting to email servers that support IPv6 (like Google).

    Do you know how I can get por 25 to use IPv6 as well on the Edge server? (so I’ll have a dual stacked environment)

    I’m a bit baffled why port 25 is not listening on the IPv6 address!

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