Have some more issues with our Exchange environment.
We have two email accounts (users), who are not getting all the emails. I have multiple people reporting NDRs when emailing them so I am sure that a "poisoned Outlook cache" could not be a problem. It does not happen all the time so I wonder if that could be specific to one of our three servers which are part of our DAG that happens to process the message. These are the last two accounts that I remember creating as well. No other users are generating NDRs.
There were more than one variants of the message so far and those were as follows:
Remote Server returned '550 5.1.1 RESOLVER.ADR.RecipNotFound; not found'
Remote Server returned '554 5.2.0 STOREDRV.Deliver.Exception:ObjectNotFoundException; Failed to process message due to a permanent exception with message The Active Directory user wasn't found. ObjectNotFoundException: The Active Directory user wasn't found. [Stage: CreateSession]'
Remote Server returned '532 5.3.2 STOREDRV.Deliver; Missing or bad StoreDriver MDB properties'