Environment
Retain 4.x
Situation
When looking at emails in Retain that are supposed to have attachments but show as 1 byte files with "Document unavailable."
Resolution
"Document unavailable" generally means that the profile's Miscellaneous tab option "Don't store any attachments, other than the message" is enabled and placeholder
Change this to "Store All Attachments." Also set the scope to "All Items in Mailbox"
Then run the job again. This will take some time as all messages will be processed again. The existing attachments will be archived and added to the messages but the placeholders will also remain.