Outlook for Mac started downloading all old emails and they are now all unread.

Hi Adam Rice, 

Welcome to Microsoft Q&A Forum!

Have a good day and I hope you're doing well!

Thank you for sharing the details of your issue. I completely understand how frustrating it must be to suddenly have your inbox flooded with over 300,000 old emails marked as unread, especially after 15+ years of smooth usage. It's disruptive to your workflow, and I'm here to help you resolve it step by step as a forum moderator. 

First, to quickly clean up the mess and mark all those emails as read without manually going through them: 

1. Open your Inbox (or the affected folder where the emails are downloading). 

2. Press Cmd + A on your Mac keyboard to select all emails. 

3. Right-click (or Control-click) on the selection and choose "Mark as Read" (or use the menu: Message > Mark as Read). 

4. This will instantly mark everything as read, reducing the visual clutter and making your inbox more manageable while we address the root cause. 

Next, since you're using a POP account (which you prefer), the issue is likely caused by emails being left on the server, leading to repeated downloads. To stop this from happening: 

1. Go to Outlook > Settings > Accounts. 

2. Select your POP account, then click on the "Advanced" tab. 

3. Uncheck the option "Leave a copy of messages on the server" (if there's a sub-option like "Remove from server after X days," set it to 1 day for safety). 

4. Save the changes and restart Outlook. 

Here are a few reasons why unchecking this should help: 

1. It prevents old emails from staying on the server, so Outlook won't re-download them as "new" items. 
2. This ensures emails are fully moved to your local Mac storage after downloading, avoiding duplicates and the constant unread notifications. 
3. It's a standard fix for POP accounts with large histories like yours, as recommended in Microsoft documentation, and it keeps your preferred setup intact without switching to IMAP. 

If you don't see this option in your settings (sometimes it can be missing due to version differences or configuration), please reply with a screenshot of your Accounts preferences, and I'll guide you further or escalate it in the forum. 

I'm truly sorry for the poor experience this has caused. Outlook should make things easier, not harder. As a forum moderator, I don't have direct access to your system or Microsoft's internal tools, but I'm committed to helping you troubleshoot. Feel free to reply with any updates or more details, and I'll be here to assist! 

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