If you've recently gotten updates to IE6 and/or have IE7, you may have also received the "new" cookie handler, popup blocker, and phishing filter. The phishing filter in particular is known to block email from specific servers that have been identified as "crime scenes," due to high volume of such mail. An innocent person using the same service can get blocked occasionally.

In IE7, the controls to turn these on/off are at Tools. In IE6 it may be at "Manage Add-ons" but I don't have a copy of IE6 to check.

It does appear that OE email "comes in" through the IE filters, although the exact dependencies of the two programs is only vaguely documented.

A service provider (ISP) can also apply filters that they don't tell you about.

Theoretically the service should pass email from anyone who's in your address book, so adding the sender's address to your book is supposed to stop them from blocking mail. OE can be set to automatically add sender's address for anyone who's mail you open and read (not really recommended, but it is an option; Automatically add addressees address is not quite so dangerous.)

If the option is set, or if you deliberately added the sender to your book, that might explain a "mysterious solution?"

John