Specific messages get stuck in the Inbox and will not move to processed nor complete the workflow

We have a recurring situation where REQAPPRV approval messages enter the Inbox for the workflow mailer and become stuck there. The mailer itself will eventually become system deactivated unless we move them out of the Inbox. For all intents and purposes, they appear to be identical to other messages that move successfully to the "Processed" folder. The only way we can distinguish them, is that they have a large size in the mailbox. Working messages are typically 3K or less and the problem messages all have a size of 37Kb. We are using MS Outlook as the client and Exchange