There is a widespread issue you may be aware of where spam or virus messages arrive with a blank or with " " for a subject and there is no sender or receipient listed. These malformed messages have the capability to "clog" Outlook users so they cannot connect to get their mail.
Many times, the Outlook user gets an error, such as this:
Receiving' reported error (0x800CCC0F): 'The connection to the server was interrupted. If this problem continues, contact your server administrator or Internet service provider (ISP). The server responded: +OK 375 octets follow.'
This is a global Internet issue and currently there is no permanent solution. For a temporary fix, we have installed a custom script which executes every one hour. It searches for these spam messages and removes them, however some messages may get past.
The problem is not with our Plesk Control Panel nor with or Qmail E-Mail Server, though most blame Plesk and / or Qmail - the problem originates with a spammer not knowing how to use his mass-mailer, and sending out a bunch of messages with that as a subject, or omitting the subject from the header entirely. There is no patch to Plesk or Qmail - as there's nothing wrong with either program - they are simply doing what they are supposed to do - which is deliver the message.
If clients are using Outlook or Outlook Express, and has certain (but not all) versions of Norton AntiVirus installed and configured to scan incoming email messages (the default configuration) then Norton hangs on the message and it hangs Outlook in turn.
For now, the BEST course of action is to login via webmail (i.e. webmail.yourdomain.com) and manually delete the offending messages that are causing the "bottleneck". These messages are easy to spot and are typically 900 bytes or less in size. After deleting the offending message via webmail, Outlook should be able to download messages.
For more info, please read the following:
Microsoft Knowledgebase Article: 813514