Hi,
Instead of trying to convert the message using adapter module I would read the message using standard configuration and then extract required values in the message mapping (for example with UDF) or XSLT from the Content field.
You will need XSD for the mail message: https://websmp205.sap-ag.de/~sapidb/012006153200000361852004E/ximail30_xsd.txt
Communication Channel settings:
Message payload:
Regards,
Radek