If you're using the Mozilla Thunderbird email program, you may have noticed that you get an error like this:
The current operation on "INBOX" did not succeed. The mail server for account <my email server> responded - nz number.
when you try to view an email with certain attachment types.
If you're also running AVG Antivirus 2011, this error is caused by the email scanning functionality of AVG. Other antivirus software might also trigger this error, but I've only ever seen it with the free version of AVG, although it's reasonable to assume that the AVG Internet Security, or other commercial versions of AVG would also cause this problem.
This can be tested by disabling incoming email scanning in the AVG interface.