Exchange 2013 – message trace shows ExplicitlyDiscarded HAdiscard on inbound missing email

I have an inbound email that a Get-MessageTrackingLog shows as arriving past the CAS server and into the mailbox server, but then shows as ExplicitlyDiscarded and HAdiscard.

RunspaceId              : f73e23f6-64e7-43b4-98bd-9871aabc5396
Timestamp               : 9/28/2015 8:53:10 AM
ClientIp                :
ClientHostname          :
ServerIp                :
ServerHostname          : MAIL5
SourceContext           : ExplicitlyDiscarded
ConnectorId             :
Source                  : SMTP
EventId                 : HADISCARD
InternalMessageId       : 47266115095789
MessageId               : <C6DDD5D4A02A3742BFD356033E939FB6029F09FAEF@VPRDMBX02.outsidedomain.local>
Recipients              : {pay.roll@mydomain.com}
RecipientStatus         : {}
TotalBytes              : 15273
RecipientCount          : 3
RelatedRecipientAddress :
Reference               :
MessageSubject          : Your mailbox is almost full.
Sender                  : Deborah@outsidedomain.org.au
ReturnPath              : Deborah@outsidedomain.org.au
Directionality          : Incoming
TenantId                :
OriginalClientIp        :
MessageInfo             :
MessageLatency          :
MessageLatencyType      : None
EventData               : {[DeliveryPriority, None], [PrioritizationReason, ShadowRedundancy], [ExternalOrgIdNotSetReason, ]}

I can't seem to find anything online other than that this is normal in deleting a shadowcopy from the primary mail server after delivering the email to the mailbox. However, in this instance there is no final actual DELIVER into the mailbox itself. The above is the final portion of the tracking log, which normally would state as "DELIVER" to the mailbox, but never got that far.

Any ideas on how to determine what happened here on why the mail was discarded before the email was delivered to the mailbox itself?