we have two exchange 2013 servers setup in DAG group with witness server.

just today I noticed that in the queue viewer I was seeing a nexthopdomain that is my secondary exchange server, type shadow redundancy.  never seen that before that I can recall, and now it has 10-20 message count.

open up the queue and the from address is healthmailboxdd something something at my domain.com

I have seen couple of messages in event log stating the periodic heartbeat between the two exchange servers failed, and "the ehlo properties for client proxy target (second exchange server) did not match while setting up proxy for user healthmailbox@mydomain.  The mismatched settings might cause some messages to get rejected. Continue with proxying even though there is a mismatch. The critical non-matching options were maxSize. The non-criticalnon-matching options were"

ran constant ping between the two servers for over two hours.  no drops, min=0ms, maximum = 2ms, average =oms.

I mean, there does not APPPEAR to be a network issue between the two systesms, they are on the same subnet, heck maybe 1 switch apart.

mailbox database copy status is healthy all around.

what would cause these errors?

Shadow redundancy messages are held for a period of time so that if there's a failure on one node, the messages in transport can be successfully delivered after a failover.  Do not worry about them.

You can change the EHLO properties in Set-ReceiveConnector -Fqdn.

I can't speak to other mismatched settings without more inform

that is good news on the shadow redundancy error, thanks.

we had just finished resolving some BackendRehydrationException errors when I noticed this error, and got concerned.