Aug 25, '06 07:30:00AM • Contributed by: killermink
Dependening on one of those annoyingly obscure configuration options and set on by default, when Outlook creates a message it stores all the text formatting information and any real attachments into a single attachment called winmail.dat. In Outlook itself, this file is decoded transparently and the messsage appears normal to the user (i.e. you don't know of the existence of the file). To everyone else, the message is actually just plain text and there is no direct access to the real attachments as they are embedded in the now visible winmail.dat attachment.
So, the first part of the hint is a warning to help prevent the build up/creation of these messages (to be fair, I am not sure if Outlook 2003 still creates them). When I first switched, I was using POP so had a lot of mail stored locally within Outlook. When wanting to move the messages to my Mac and searching this very site for the best method, I opted for the seemingly easiest method of using an IMAP server as a transport - i.e. copy from my Outlook local folders to an IMAP remote folder then download to Mac using the same IMAP connection. This is still a sound theory, but unfortuantely any message in Outlook that is in the winmail.dat format will remain like that after the transfer (and it seems impossible to know which). Therefore, when complete, you will likely have many messages on your Mac where access to the real attachments is lost, as they are actually embedded in the previously invisible winmail.dat file. (Also, any mail that had no real attachments originally but even minor formatting (such as a bolded word) will now have a single attachment just for the sake of containing information on formatting.) Once you have these, to view the real attachments you have to get other programs to decode the winmail.dat afterwards, which is not really very convenient plus requires a third party application. So, the warning here is to not use Outlook to transfer messages via IMAP.
So, how should you transfer these messages if they are in Outlook already AND in the wrong format? Turns out that Outlook Express, which should be on every machine that has Outlook, is also able to decode the winmail.dat messages transparently like Outlook, but it does not create them! So, if you import the messages into Outlook Express from Outlook they will appear perfectly. More importanly, when using Outlook Express to transfer via IMAP it leaves them in the decoded state so the messages will have ALL the correct formatting, and ALL the real attachments!
The above sounds sensible enough, but the real benefit of the process (and the beauty of this hint that I could not find anywhere else on the interweb) is that if you have already mistakenly moved a bunch of mail using the Outlook IMAP method previously, and now have a bunch of mail with winmail.dat's, you can use this method to convert them back to their original format! Simply put them back onto an IMAP server, connect to that server using Outlook, then copy the mail locally. They will now be in Outlook and will look correct once again due to the auto decode. Now open Outlook Express and import the mails in question from Outlook. Finally, use an IMAP connection in Outlook Express to copy back to the server. Outlook Express will create decoded messages on the Server and all the winmail.dats will be gone forever!
