This week, we shipped the February Cumulative Update for Outlook 2010. One of the fixes in there relates to the MAPI<->MIME conversion API. In particular, with Outlook 2010 if you use MAPIToMIMEStm to convert a message to EML you’ll see the source message get updated. This didn’t happen with Outlook 2007. The reason for the change in behavior has to do with how we handle conversation threading, particularly for messages that don’t already have some of the conversation properties already stamped on them. In order to preserve threading should the MIME message come back in to MAPI (a scenario which occurs with POP3/SMTP profiles and replies), we would stamp the conversation properties on the source message during the export.
In general, this is fine, but it did cause a problem for some vendors who needed their export to not modify the source. So for them, in the February CU we enable a new flag:
#define CCSF_PRESERVE_SOURCE 0x40000 // don't modify the source message
Pass this new flag in to MAPIToMIMEStm and we’ll bypass the new code that saves changes back to the source message.
One caveat: We may still make changes to the message. This flag skips the SaveChanges call that commits the changes. So if your code subsequently calls SaveChanges, you’ll see the changes made during the export get committed. Of course, if your code is calling SaveChanges, you probably won’t be using this flag in the first place.