Ha! I think we have it!

I accepted the invitation, then exported it twice: once to CSV (Windows) and once to CSV (DOS). They seem identical in both CSV (viewing in Notepad) and when loaded into Excel. I wondered whether you were using a different export format than I, but they're the same, so that's one theory down.

In Excel, the required attendee (my email address) is in col 11, and the optional (your address) is in col 12. Neither one has quote marks, of course. But when I open them with Notepad, both addresses are in double quotes.

You haven't yet made the distinction between how they look in the CSV and how they look once they're read into Excel, so I suspect you haven't noticed it yet. If you already know this then don't take offense, but just in case you didn't: A CSV file is a plain-text "unload" (so to speak) of the contents of a workbook. When you double-click on a CSV document, Windows knows it's a kind of Excel workbook and by default (unless you change the association for the .csv extension) uses Excel to display it. But you can right click on a CSV, Select "Open with" and point it to Notepad; or you can open a Notepad window, then drag-and-drop the CSV icon to that window. This lets you look at the native text of the file. There you'll see something like this:That's the appointment you sent me, you see, except that I sanitized your email address in case you didn't want it seen here.

When I first brought up the issue of quotes, I was wondering whether the email addresses were quoted in your CSV. (They wouldn't show the quotes in Excel, of course; they're not needed there.) The fact that you never said "Notepad" while telling me what you were looking at in the CSV makes me suspicious that you didn't know what I just explained above. But I think we have the scoundrel now, so don't just read about it: I want you to go try it. Do it now: Use Notepad to look at the invitation that you exported from Outlook. (For future reference, I'll pretend this file is named "Outlook.csv". In Notepad, the email addresses have double quotes around them; confirm that this is true for you as well.

Next, double-click on the CSV normally and look at the invitation in Excel. The quotes around the text fields don't show up in Excel; that's normal.

Now: In Excel, use Save As to save that file in the CSV format. Make it a different file name than the one you imported, say "Excel.csv".

Last step: Look at Excel.csv in Notepad. Notice: The email addresses don't have quotes around them.

Voilà: We now see the problem. Well, I think we see the problem. When Outlook exports appointments, it puts the email addresses in quotes; my hypothesis is that it expects them to be in quotes during an import, too. But Excel sees no need to put quotes around character values that have no spaces or other special characters in them, so when you export your data from Excel to a CSV, the addresses aren't in quotes, and—I expect—that's causing Outlook not to understand what it sees.

You can test this by using Notepad to put double quotes around those addresses and then importing Excel.csv into Outlook. If it works correctly this time, you're on your way; you just gotta figure out how to get Excel to use quotes in those columns. I'm not sure Excel offers an easy way to do that, but it may; and if it doesn't, there are certainly ways to get around it.

Io, Io, Io, Pæan!