Quote from: tino – multipart should mean that there is both a html and text version or that an additional attachment is sent along with the text.
That was both Spuds and my assumption also... but Pegasus for one won't include plain text in "some" versions of multipart, if you have "rich text" (which is their plain text/html switch) selected. Other aspects of multipart, it will include both. No idea what, if any of it, is RFC compliant. But if it works, well, I haven't heard the FBI is investigating RFC standards violations ... yet anyway.
Sometimes I wonder if there is email traffic out their being sent around so intentionally "malformed" they get rejected by every standards-compliant or semi-compliant parser except the clandestine servers they're intended for, so hardly anyone knows about them. An Email "Dark Web" so to speak. Nah, I'm sure that's just "crazy talk"..