Event id 1025 SMTP rejected a (P1) mail from 'HealthMailbox...@domain.local' with 'Client Proxy SRVEXCHANGE01' connector and the user authenticated as 'HealthMailbox...'.

Hi

I have Exchange 2010 and 2013 co-existance and on the 2013 server I'm getting an event id 1025 every 5 minutes saying:

SMTP rejected a (P1) mail from 'HealthMailbox21b2b04b53c34b6fae8a64de76ebb6d7@domain.local' with 'Client Proxy SRVEXCHANGE01' connector and the user authenticated as 'HealthMailbox21b2b04b53c34b6fae8a64de76ebb6d7'. The Active Directory lookup for the sender address returned validation errors. Microsoft.Exchange.Data.ProviderError

I have deleted the health mailbox account on ADUC and restarted the Exchange health service. The accounts are recreated but I still get the same error. The UPN is correct as it is using the internal domain name and the account exists on the "Monitoring Mailboxes" OU. 

I also get another error regarding a monitoring mailbox on "Microsoft->Exchange->ManagedAvailablity->Monitoring":

Any ideas?