I have a dozen different email accounts for different roles, all of which I interact with through mail.app. I had a hard drive disaster, and restored from TimeMachine. It did an impressive job of restoring my complex email structure for backed up email, and of course new emails coming in are fine. But I am missing the month (urgh) of emails between the most recent backup and the restore. They are still on gmail (some POP and some IMAP accounts). How do I mark them as not already downloaded, so that they'll be retrieved from gmail?
For the mailbox in question, in the Mail.app, go to
Mailbox (top menu), and then select
Rebuild. You may need to relaunch the app for the older messages to be downloaded.