Posting as a reply to hopefully preserve formatting:
Adding .eml and .msg is not solving the issue when email has another email copy-pasted into it - the issue described by the original poster above. ST blocks those with a "The user attempted to send a file through email with an extension that is not allowed: Attachment 1" note even when we have .eml and .msg extensions enabled.
Gmail shows those as noname.eml but in fact there is no file name for the email message embedded into the email message:
Content-Type: text/html; charset="utf-8"