Error occur in triggering email receive message event in 18.2

Hi

We recently updated to version 18.2. Pre upgrade, we have existing integration with our email server where one of process models in our application is triggered based on incoming email messages (Receive email and at Start Event). Pre upgrade to 18.2, it was working fine based on configurations of combinations of ejb-jar, custom properties and password properties.

However post 18.2 integration, the incoming email message that is supposed to trigger the process model is no longer occurring. Below is the error extracted from server log :

01:00:39,768 INFO [stdout] (Appian Work Item - 999906 - execution02 : MessageEventRequest) 2018-09-12 01:00:39,768 [Appian Work Item - 999906 - execution02 : MessageEventRequest] ERROR com.appiancorp.messaging.MessagePublisherServiceImpl - Message rejected due to missing Destination [ID=537037338, MessageType=Process to Process, UserName=<username>R, DestinationPMUUID=null, DestinationPMID=null, DestinationProcessID=null, DestinationEventPersistentID=null]

In the custom.properties file, there is no setup using Destination Key , instead we use alias as  per below. 

conf.mailhandler.alias.<mailbox name>=processmodeluuid<uuid>

Can anyone please help with this error? At this stage, we are unsure if this is due to version upgrade which has different mail server setup configuration in comparison to our previous version 17.1.

Thanks 

Julie