This is a known Outlook issue. The Outlook program handles IMAP poorly, at best. Thunderbird - or any other email program for that matter - does a much better job with IMAP.
And this is a result of the amount of email in your IMAP account growing large, and Outlook not being able to handle it properly.
Short of switching to a good IMAP email program, try the following:
- Click on "Tools", "E-Mail Accounts", "View or Change existing e-mail accounts".
- Select your Brownrice Email account and click on "Change".
- Click on "More Settings" and finally on "Advanced".
- Change the "Server Timeouts" to the maximum (10 minutes) and click on "OK", "Next" and "Finish" to save your changes.
- Exit and then restart Outlook.