Emails being Explicitly Discarded in Exchange 2013 DAG (HADISCARD)

I am seeing messages Discarded after being delivered to 1 DAG member, and then transferred to the other (The one containing the active mailbox)

I have a NLB doing round robin. Here's the scenario:
UserA is on DBA which is active on EXA
The email comes in and gets delivered to EXB via the NLB
EXB then forwards/transmits/replicates the message to EXA
For whatever reason EXA discards the message

Any ideas why this is happening? This is what I'm assuming is happening. Please correct me if I'm wrong.
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Aurel VacalieCommented:

Do you use custom receive connector ar the default ones?
If custom, check the limit for number of messages for one source.
tzadminAuthor Commented:
I set it to 100 which seems to be the max. What is the default receive connector set to? I assume it's also 20 which means I would have had the same issue.
What happens if I'm using a security appliance? All connections would be coming from one place. This limitation doesn't make sense to me.
tzadminAuthor Commented:
I believe that the Exchange servers use the "Default <Servername>" connector to intra-org transmissions which is set to Unlimited Connections from a single source by default.

The HADISCARD messages I've been seeing appear to be from the Shadow Redundancy process.

HADISCARD: A shadow message was discarded after the primary copy was delivered to the next hop.

I don't see anything in event viewer about messages being rejected from MSExchangeTransport so I don't believe there's an issue with my connectors.

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tzadminAuthor Commented:
Not a receive connector issue although the information is still valuable.
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