More Information
The WebDAV protocol can be used to send emails. This can be done by creating the email in a mailbox folder (i.e. the Drafts folder) and moving this item to the ##DavMailSubmissionURI## folder. This is described in this MSDN article: Sending a Message (WebDAV).
Attachments can be either added to this mail using the Outlook Web Access method, or by issuing a PUT, using the address of the mail as folder: If the address of the item is
http://myserver/exchange/jdoe/drafts/new_mail.eml
the url for the PUT command would be something like http://myserver/exchange/jdoe/drafts/new_mail.eml/attachment.zip.
If you created the mail item via the PUT method, instead of a PROPPATCH, you might get a 409 - conflict error when adding attachments to the item.
Solution
A workaround seems to be to issue a PROPPATCH on the mail item itself. The PROPPATCH does not necessarily have to change the item. It's more like a touch command. After this PROPPATCH command, the PUT command for the attachment will be successful.
Status
The status of this problem is unknown.