It depends on the message you examine.
The problem is that many bounces don't follow multipart/report, message/delivery-status format. They are just plain text emails. Mail.dll does it's best to process them. However sometimes it is impossible.
There are several reasons that can cause this:
- Maybe the reason was not provided at all,
- maybe it is impossible to extract it, because the bounced message is using regular text and not structured data (such as multipart/report, message/delivery-status)
- or maybe the bounce message is using an unknown format.
If you believe it can be extracted, please send us the raw eml version of this message for examination to the support email address, you can find in this page footer.