Messages with XML content in nabble does not contain XML on mailman list

[52n]

Hi there,
I am using nabble to provide an additional view to our mailing list driven by mailman 2.1.15.

My problem is that XML content that users provide via the nabble UI is not send to the mailing list.
Example:
***
My message with example XML:
<xml>
  <innerXml />
</xml>
End of message.
***

On the mailing list, we receive the following message:
***
My message with example XML:

End of message.
***

Is there any setting in mailman or nabble that fixes this problem?