Do not send pre-message if post-message sending failed

Right now, it can happen that the pre-message goes out fine, but then sending the post-message fails, usually because it's too large. For the receiver, this looks very weird because it looks like a message with attachment arrived, but when trying to download it, it stays in "Downloading..." forever.

The solution is to send the post-message before the pre-message, and then delete the pre-message smtp entry if sending the post-message fails.

We could think of renaming the pre-message to preview-message, but the name post-message needs to stay now because it is used in headers.