This is a staging table with an entry for each message of each mailbox currently being imported in a controlled way by the user interface. The contents are meant to be deleted soon after the import is complete.
Structure of import_message
Number of the message in the mailbox starting from 1.
The actual message content.
'Y' if the filters for incoming mail should be applied when importing this mailbox, 'N' or NULL otherwise.
Status of the import for this message. Values are 0=new, 1=imported, 2=cancelled.
The ID of the message once it's imported.