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

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

First reported by MVP Norbert Klenner, details of the have been added to the release notes for Exchange 2016.

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

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

When an 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 errorRecipient not found by SMTP address lookup. The recipient won’t receive the message.

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

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

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

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

Set-RecipientFilterConfig -RecipientValidationEnabled $False


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