If you attach one email to another, you might get this bounce message (aka “NDR” or Non-Deliverable Report):

John Doe (john.doe@domain.com)
The e-mail system had a problem processing this message. It won’t try to deliver this message again.

Diagnostic information for administrators:
Generating server: MAILSERVER.domain.com
john.doe@domain.com
#550 5.6.0 M2MCVT.StorageError; storage error in content conversion ##

You probably have Outlook configured in “online” mode. In other words, you don’t have caching turned on. The real problem is that the server is having trouble retrieving the original message and attaching it to yours while the original message is still stored on the server. To fix this, you can do one of two things:

1. Enable caching in Outlook. This could take some time, depending on what version of Outlook you are using and how large your mailbox is.
2. Drag and drop the original message out of your mailbox onto your desktop or elsewhere on your local computer. Essentially, this is like caching just the one message.

Then you can re-attach the cached message to your original email and send it successfully.