Exchange trying to resolve external e-mail addresses in local Active Directory

Hi

On all of my Mailbox Database servers, i'm getting the following warning in my Application log: 

Level: Warning
Source: MSExchange Mid-Tier Storage
EventID:  2009
Message:
[Process:w3wp PID:6032 Thread:89] Error occurred while resolving the Active Directory object for from email address field: 'user@externaldomain.com'. Audit log will not be generated for this case. Exception details:
Microsoft.Exchange.Data.Storage.ObjectNotFoundException: The Active Directory user wasn't found.
   at Microsoft.Exchange.Data.Storage.ExchangePrincipalFactory.FromProxyAddress(IRecipientSession session, String proxyAddress, RemotingOptions remotingOptions)
   at Microsoft.Exchange.Data.Storage.ExchangePrincipalFactory.FromProxyAddress(ADSessionSettings adSettings, String proxyAddress, RemotingOptions remotingOptions)
   at Microsoft.Exchange.Data.Storage.ExchangePrincipal.FromProxyAddress(ADSessionSettings adSettings, String proxyAddress)
   at Microsoft.Exchange.Data.Storage.COWAudit.GetSubmitEffectiveMailboxOwner(MailboxSession session, CallbackContext callbackContext)

I have three Exchange Server 2013 MBX/CAS servers, and two Exchange Server 2013 Edge Transport servers in front of them.

As mentioned earlier, this warning is on all three of the MBX/CAS servers. The external e-mail address vary.

I've used the Get-MessageTrackingLog to debug, and I can see that this error comes when an internal user sends a "Meeting Forward Notification" to an external e-mail address. Exchange tries to resolve the external e-mail address in Active Directory and throws this warning, for some reason.

Is there anyone that knows how to fix this?