Microsoft Outlook 2003 Error 0x8004DF0B has been plagueing me for a long time. I have a couple of clients using Outlook in a bunch of different offices, and connecting into more-or-less out-of-the-box Courier IMAP setups on FreeBSD, with client SSL enabled for both inbound (imaps) and outbound email (both smtp with starttls and Outlook's poxy smtps/465).
The problems started when I installed NOD32 Antivirus. On a fairly regular basis, the Outlook installation would start reporting the following error:
Task 'Checking for new mail in subscribed folders on blah.domain.com.' reported error (0x8004DF0B) : 'Outlook is unable to download folder (null) from the IMAP e-mail server for account blah.domain.com. Error: The connection to the server is unavailable. Outlook must be online or connected to complete this action. If you continue to receive this message, contact your server administrator or Internet service provider (ISP).'
Disabling NOD32's email checking system invariably fixed the problem. However, NOD32 believes that this is a mistake and every time it performs an update, it would automatically re-enable email checking (this is just stupidly broken). However, it appeared for a long time that the problem was related to the anti-virus system.
The problem turned out to be that Courier IMAP has a per-IP address limit of 4 connections. And as several of these clients were in the same office, Courier just started abruptly terminating connections. Outlook couldn't deal with this properly, and instead of providing a usable error message, it printed out 0x8004DF0B.
The fix: Add the following line to the imapd-ssl configuration file:
...and restart imapd-ssl. Blindingly and irritatingly obvious in retrospect.