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 Thursday, July 09, 2015 1:01 PM

Only one copy of the email is being sent. They have different message ID's.

I've compared the headers on the emails, these make it clear that the duplicated email received later is the copy that was made on the other server for shadow redundancy. It looks like it is this copy that is being delivered at a later date, rather than being discarded as it should be.

I did make some progress last night and may have resolved the issue but I'm not 100% sure of the explanation. I noticed that at the time the duplicate emails were delivered, when the mail.que was being re-created, the message tracking logs increased substantially in size. Appears this was due to messages related to our journaling mailbox being full.

I've increased the size of the journaling mailbox and suspect the issue will be resolved but I'm not sure why a full journaling mailbox would result in duplicate emails being delivered?

Only one copy of the email is being sent. They have different message ID's.

I've compared the headers on the emails, these make it clear that the duplicated email received later is the copy that was made on the other server for shadow redundancy. It looks like it is this copy that is being delivered at a later date, rather than being discarded as it should be.

I did make some progress last night and may have resolved the issue but I'm not 100% sure of the explanation. I noticed that at the time the duplicate emails were delivered, when the mail.que was being re-created, the message tracking logs increased substantially in size. Appears this was due to messages related to our journaling mailbox being full.

I've increased the size of the journaling mailbox and suspect the issue will be resolved but I'm not sure why a full journaling mailbox would result in duplicate emails being delivered?

• Marked as answer by Jon7219 16 hours 55 minutes ago

Hi Jon,

Try to do a message Tracking to identify which server created the duplicate mesage using he message IDs.

We haven't had any further occurrences, so it appears the journaling mailbox running out of space was the cause.

Not sure why it would cause this behaviour though.