> the RFC specifies that the BOM should prefix only unstructured
> message part, but current Python implementation puts it in the
> middle of structured part, thus confusing RFC-compliant receivers.

How do you work that out? The implementation puts the BOM before the message part:

        prio = '<%d>' % self.encodePriority(...)
        prio = prio.encode('utf-8')
        msg = msg.encode('utf-8')
        if codecs:
            msg = codecs.BOM_UTF8 + msg
        msg = prio + msg

Perhaps I've misunderstood you or the RFC5424, but I don't quite see how. I'll mark this as "pending" and "invalid" awaiting a clarification from you (ideally with a specific example).