How do I make Outlook leave a copy of my messages on the server?

Knowledgebase

Configuring Outlook to Leave Messages on the Server

If you use POP3 in Outlook, messages are downloaded to your computer and removed from the server by default. This means you cannot see them in webmail or on other devices after Outlook downloads them. Here is how to change that.

Outlook for Windows

Go to File > Account Settings > Account Settings
Double-click your email account
Click More Settings
Go to the Advanced tab
Check "Leave a copy of messages on the server"
Optionally check "Remove from server after X days" to prevent the mailbox from filling up
Click OK and close

Better Solution: Switch to IMAP

Instead of using POP3 and managing the "leave on server" setting, we recommend switching to IMAP. IMAP keeps all messages on the server by design and syncs seamlessly across all your devices. Remove the POP3 account from Outlook and re-add it as IMAP using server mail.yourdomain.com, port 993, SSL. All your emails will remain on the server and stay synchronized.