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.
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 errorRecipient 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:
- Disable recipient validation on Exchange 2016 CU1 Edge Transport servers
- Route inbound email to an Edge Transport server that has not been upgraded to Exchange 2016 CU1
- 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: