Hi,
We got an application sending out emails internally to our end users, it does so in bulks of about 50 or so. The application sends via a Exchange 2013 receive connector where the application server is listed as an allowed relay.
About 30% of the emails go through fine, but most are dropped in Exchange as such:
TimestampSourceContextSourceEventId
2/04/2015 13:02ExplicitlyDiscardedSMTPHADISCARD
Its different recipients that get dropped every time and no other issues sending to any of these users.