We have some POP users on our Microsoft Exchange 2000 Server system. When other users send meeting requests to the POP users, the meetings show up as attachments with .ics extensions. What's causing this behavior and how can we fix the problem?
This problem occurs because Exchange is trying to be a good Internet citizen. In the past, Exchange sent meeting requests as rich-text items, which were derived from the Messaging API (MAPI) contents in the Exchange Store. This approach worked for Exchange organization invitees, but the meeting requests appeared as weird binary junk for Internet recipients.
To fix that problem, Exchange 2000 converts meeting requests to the Internet-standard iCal format, resulting in the .ics attachment. This conversion happens when the email client retrieves the message, so POP clients end up with the .ics file instead of a typical meeting request. No workaround exists for this problem except to request that your POP users stop using POP.
—Paul Robichaux