Review of a given worker log can show the account which is failing. Using the example above you have to determine the Process and Thread IDs of the action being taken:
2015-03-20 00:10:59Z|ERROR|P:1460|T:5|FolderCreator|Unable to create/retrieve folder [Archive Folders\Sent Items] in [ArchiveMsgFolderRoot] for [User@Domain.net]
In this case these are 1460 and 5 respectively. Then following the ID pair up the log, you can find what ingestion account was used for this communication. Below is an excerpt showing mention of this log entry:
2015-03-20 00:10:55Z|INFO|P:1460|T:5|WrappedServiceCredentialStorage|Using [email@example.com] as Service Ingest account
In this case, attempting to access Office 365 using this account name and the same password supplied to MMPST will also fail. This is at least one instance of an ingest account failing authentication to the target. You may need to continue to review the instances of failures to ensure all the ingest accounts have been properly configured in MMPST and Office 365.