We recently completed migration from SBS 2008 to Server 2012 R2 with Exchange 2013 CU7.
Old Exchange was uninstalled and demoted.
Every 5 min we getting an error in Application Event Log:
Event ID: 1025
Source: MS ExchangeTransport
SMTP rejected a (P1) mail from 'HealthMailbox065a4231a8a6475985d8d6a41f5b4c06@domain.local' with 'Client Proxy Server' connector and the user authenticated as 'HealthMailbox065a4231a8a6475985d8d6a41f5b4c06'. The Active Directory lookup for the sender address returned validation errors. Microsoft.Exchange.Data.ProviderError
There are 14 HealthMailboxes account listed in AD including one listed in this event but only this one generates errors.