A few weeks back we started getting duplicated emails delivered to users, I'm struggling to figure out the cause....

• We have two Exchange 2013 servers in a DAG, SP1 with no CU's applied. I'm not convinced it is configured correctly, but it has been working ok up until this issue started. I'm not aware of anything being changed that could explain/have caused this issue.
• When an affected email is sent it is received by server01 then copied to server02 for high availability. It is the copy on server02 that is then re-delivered a number of days later causing the duplication.
• It appears to only be affecting internal emails sent via an anonymous relay.
• The duplicated emails occur every 4-5 days, it happens in a big block then nothing until the next occurrence.
• The duplicated emails appear to tie in with a new mail.que file being created by Exchange on server01.

Anyone have any ideas as to what is causing this and how it can be resolved?

• Edited by Jon7219 18 hours 28 minutes ago

Hi,

Please check the sender sides Sent Items to see if the sender sends multiple times.

Check the message tracking logs on both mailbox and Hub servers to see:

1. If those duplicate messages have same message ID or different message ID.

2. How many SUBMIT event for those same items.

And how did you configure the anonymous relay?

Best Regards.