I don't want to think about how many email messages I get a day, but in 2010 it was over 10,000 messages that I didn't delete on sight (or didn't get around to deleting during the year). I know that because while I've never declared email bankruptcy and deleted the lot (old messages are far too useful), I do like to start the new year with at least the feeling of clearing the decks. My equivalent of email Chapter 11 is to empty out my inbox by stuffing the old messages into an archive folder late in January. If I haven't read your message before then (and there are 1,303 messages that I haven't read more than the subject of in there, which at least is better than the 2,173 I didn't get to in 2009), then at least it might come up in a search.
I don't use Outlook's Auto-archive because I don't want the messages compressed in a separate file; I just want to know that when there are 569 unread messages in my inbox as there are today, they're messages from this year rather than five years past. I don't even do a fancy date search (I really want the next version of Outlook to have a date picker for searching between two set dates). I just collapse the chunks of time in the inbox (today, yesterday, last week, ever such a long time ago) until I can easily grab a year's worth of mail, use the Move To command in case my hand slips when I drag the messages into the new folder and do something else while Outlook tidies it away. I don't worry if I get the spinning frisbee of doom (as I call the Windows 7 'I'm busy' cursor); Outlook chugs away for a while regardless.
But this time the chugging was more pronounced, I was seeing strange error messages and everything ended up in my Deleted Items folder. I didn't mind moving them back out of there but again Outlook started hanging if I moved more than a couple of hundred messages at a time and if I selected too many, I'd see that odd message: "creating a new item from these messages could take some time'. I didn't want to make a new item, I wanted to move the items I already had. Once I clicked Yes instead of No, briefly saw a new email with a vast number of attachments and then Outlook crashed.
We wondered if the problem was trying to do it on a slow wireless connection rather than in the office, but I tried when I wasn't even online with the same result.
Moving messages a chunk at a time, I took the opportunity to delete the odd thing like spam, bounce messages and out-dated appointments: Outlook deletes invitations once they've been accepted but I seemed to have a few left around from last year, usually when there's a message in the appointment declining the meeting or suggesting a new time. And then I noticed I had an appointment in the group of messages I’d just tried to move - and there was the message about making a new item. My theory is that when you ask Outlook to move a mix of messages and appointments into a folder it attaches them all to a new message; why, I have no idea, but it explains why selecting large numbers of messages produced the dialog - I was seeing it every time I selected enough messages to include one of the lurking appointments. And the crash was because an email with a couple of thousand attachments is more than I expect any email package to handle gracefully).
My next thought was to search for the appointments and either delete them or move them together (because moving appointments doesn't always make a new item and if it did there wouldn't be enough to make Outlook hang). But how do you search for an appointment? I can choose a specific appointment field to search in from the ribbon (click the green More button and choose Add Form Field to pick from every single field in every Outlook form) but I couldn't find a way to search by type. Then I accidentally right-clicked on the Arrange By: heading in the inbox and was reminded that I could just sort by type. That put all the appointments at the top of the list and all the replies at the bottom (with mail messages in the middle because it's alphabetical by type).
Once I deleted or moved the entries that weren't email I was able to move the messages without any more problems; when Outlook isn't trying to stuff thousands of messages into the attachment field, it takes only a couple of minutes to shuffle them into the right folder.
At this point I switched to a PC with an older copy of my mailbox that hadn't been connected since December 31 and tried the steps again. Selecting any number of messages from one to hundreds at the same time as a meeting message always brings up either a new message with everything attached or a warning about how long that will take. Deleting or moving those meetings let me move 10,000+ messages into a new folder in about 90 seconds. Checking the numbers in each archive, the older copy of the mailbox had some 2,500 more archived messages than the one I'd been working with originally; that sounds about right for the number of attachments that crashed Outlook and neither the partially created new item nor the original messages (which were, at this point, in the Deleted Items folder because moving them out was what I was struggling to do) survived the crash. This is why we take backups, after all.
To make sure that when I sync all my different PCs I get the full copy of the archive rather than Outlook helpfully deleting those messages on the other machine, I saved the folder to a PST. This takes a lot longer than moving messages from one folder to another, which is just changing metadata (well over half an hour in this case), and it's not immediately obvious how to do it. On the File menu - which Microsoft calls the Backstage menu but is labelled File in Outlook - choose Account Settings > Account Settings (yes, twice); pick the Data files tab and click Add. Name the PST file and choose where to put it then go back to the folder you want to add to the PST. Right-click and choose Copy Folder and pick the PST from the folder list; you can just switch the Navigation Pane to show all folders and drag it into the PST but given how long it takes to complete I didn't want to risk dropping it in the wrong place. If you need to get the messages back out of the PST later and it's not showing up in the All Folders list, click the Open File Location button on the Data Files tab to get it back.
So, on to another year of not quite keeping up with my email…
UPDATE: The Office team confirms that it is the mix of appointment and meeting requests selected that cause this message. In fact there's now a KB entry to explain it, with a concise description of the workaround.