This question is somewhat of a long shot, but I've spent hours on it to no avail. I have some code that generates an email file on a webserver, and allows the user to download that email and open it in Outlook. From here, they can make various manual changes to the email before they send it to a bunch of people.
Right now, I generate a .OFT file, which is basically an email template. What I want to do is generate a .MSG file, which is an actual email. From a binary point of view, it seems these file formats are identical. They have the same Stream IDs and properties and stuff.
My approach was to first create a blank email message in Outlook and then just save it to a file called Base.oft. In my code, I open the document and modify Stream ID __substg1.0_1013001E which is the ID for the HTML email body. I then save the file and write it out to the cilent. This works perfectly.
I tried the same approach with the MSG format. I created a blank email message, saved it as Base.msg, and modify the same Stream ID. If I look at the resulting file, the new body is actually in there and saved. However, if I open the email, the body is still blank.
What's even weirder is if I type in a body in Outlook and save that to the base file, I can see that body under stream 0_1013001E. If I then modify that stream with a different body, I can verify the new body is indeed saved in the file, but if I open the message in Outlook, I see the old, original body. It's as if the email body is stored in a different place in the file for the .MSG format, however I've looked through each stream and cannot find anything else that looks like it could be an email body.
Perhaps .MSG files are encrypted, or their bodies are stored in some proprietary binary format unlike .OFT files? Hopefully someone has some insight on this, as I scoured the Internet and found basically nothing on these formats.
Update:
It seems the .MSG format stores the body in Stream ID __substg1.0_10090102 - Which is encoded in some binary form (not sure what.) If I delete the stream (or set it to a single \0, the file becomes corrupt.
First of all, to find more information on this and related topics, move away from raw substream numbers and google for the corresponding MAPI properties. For example, 1013 is PR_HTML and 1009 is PR_RTF_COMPRESSED. MAPI has ways of synching the body from one format to the other.
See this article on MSDN for a good overview of all content-related MAPI properties (i.e. the different "streams" inside the .MSG file).
To write PR_RTF_COMPRESSED, wrap the stream inside WrapCompressedStream. On the other hand, in your particular situation you might want to avoid the MAPI-dependencies in your code, so maybe you're better off finding the PR_STORE_SUPPORT_MASK and setting the STORE_UNCOMPRESSED_RTF bit. This will allow you to use straight RTF in the PR_RTF_COMPRESSED substream. Or Outlooks fancy html-wrapped-in-rtf, if you are feeling brave.
None of this stuff is for the faint of heart, but seeing how you are already handing raw .MSG substream writing, I'm guessing it would be feasible.
When it comes to the format, there is no difference. the only difference is that OFT files have CLSID_TemplateMessage ({0006F046-0000-0000-C000-000000000046}) as the storage class (WriteClassStg), while MSG files use CLSID_MailMessage ({00020D0B-0000-0000-C000-000000000046})
来源：https://stackoverflow.com/questions/7957827/is-there-a-difference-between-the-outlook-msg-and-oft-file-formats