A significant part of weirdness involving outlook/exchange is that
it is not an SMTP-based email clientEven though X.400 is no longer officially feature of Exchange, the entire data model of MAPI is based on it, shared between Outlook and Exchange, with somewhat lossy translation when it has to go outside of X.400-over-RPC that MAPI provides.
Sometimes you can get burned by vestigial parts of the model, like how MS MAPI implementation as provided by Outlook/Exchange (there used to be others!) does not actually support HTML email, and crashes with corrupted message errors if given an email object containing a HTML body.
Now I can hear you going "but Outlook does HTML email!".
Outlook converts HTML email body part into RTF-wrapped HTML and stores the resulting message in RTF body field of message object. In fact, before Outlook got changed to convert RTF to HTML, Outlook users were infamous for sending RTF formatted emails (at least RTF was always documented, as it was supposed to be interchange format).
But MAPI messages do also contain a HTML message body field... But if you put HTML there, MAPI.DLL explodes - or at least did every time I did it.