Microsoft Outlook and Azure logic apps interpret e-mail in a very strict manner.

I ended up using this resource to understand some aspects: https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/message-format-and-transmission

What I had trouble with was the strict handling of e-mails being sent by a black box type of software run by another party. They would send an e-mail with text in the body and also have an attachment of the same content. The e-mail had the body text encoded with the same type as the attachment: Content-Type: text/xml

Outlook and an Azure logic app were strict and interpreted this as an attachment even though it was where the body of a message would go. They auto assigned a filename to the attachment. Thunderbird and Gmail were more relaxed in the handling of the e-mail and when viewing the body section (even through it had a text/xml type, because it was where the body would be, they were more user friendly and interpreted it as plain text and just displayed the XML as the body of the message and then the attachment was handled correctly as an attachment.

My take away is: It appears Microsoft products are very strict in how they handle messages.