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.