Usually messages come through the sb_server POP3 proxy OK, but it seems that certain messages cause sb_server to hang and the client times out, e.g. using Outlook 2007 the status bar shows "Downloading Message 4 of 4 (9.44Kb / 279Kb)" if the message was one of a list of emails to download, or just sticks at "Send/Receive Status 83%" if the blocking email is the only one to download. Eventually the client times out:
Task 'Default - Receiving' reported error (0x8004210A) : 'The operation timed out waiting for a response from the receiving (POP) server. If you continue to receive this message, contact your server administrator or Internet service provider (ISP).'
This also happens when using Outlook Express, and only seems to be with certain emails. The way I have managed to reproduce it is to send a large HTML email with a Word document attached (approx 200Kb email size total) to my account from another email address.
The only way round this is to bypass sb_server to download the message directly from the server and then switch back to using it after the email has been removed from the queue.