Use:

~userqueuemessage~

Required parameter: Account, Msgid
Optional parameter: Folder ('Inbox' is used if not specified)

You can also use the :<max size> modifier to set the maximum size of the response. If none is specified then a maximum size of 1000 characters is used. (e.g. use **~userqueuemessage:10000~)

This returns the specified message from the user's mail folder. The message is returned in 'raw' format, for diagnostic purposes.

The returned data is HTML encoded with some formatting in place