Unfortunately GroupWise doesn’t expose the MIME representation of every message – most internal messages, for example, won’t have a MIME representation.
Access to MIME data for messages that do expose them is limited. The MIME data needs to be accessed via the MIME.822 attachment where they exist. You need to save the attachment to disk, then manually process the file. Note you cannot edit a MIME file of a received message then re-attach it to the same message. You would have to re-create the message from scratch – a non-trivial excercise.
None of these limitations are related to Formativ – this is just the way the GroupWise client works.