In testing another change in the caching code, I switched this from TRUE to FALSE.

When it was TRUE, the inbox was not setting messages as being read.

When I turned off caching, it started working normally.

Question about this though: it says when TRUE, cache messages & mailbox data in local database. When this is FALSE, does this make it cache to the TEMP directory, or instead it just stops caching at all?

// enable caching of messages and mailbox data in the local database. // this is recommended if the IMAP server does not run on the same machine $rcmail_config['enable_caching'] = FALSE;