In what scenario would this be a boundary? I have looked at a few different emails from the same source and this appears on only one but there is nothing that is really different, all have attachments. I tried to duplicate by sending and adding attachments but was unable.
It is preceded by mixed, and followed by alternative/plain/html
There is an RFC about the Multipart/Related Content-type. It’s RFC 2112 and likely the best source to find answers to your questions.
Microsoft’s document here also gives a good summary:
multipart/related:
The multipart/related content type is used for compound documents (that is, messages in which the separate body parts are intended to work together to provide the full meaning of the message). Additionally, multipart/related can be used to provide links to content not contained within the message. Multipart/related can be used for compound documents where the object is built progressively from pieces, starting with the “root” body part as specified in the start parameter. If the start parameter is not specified, then the first body part is considered the starting point or “root” body part. Multipart/related requires a type parameter. The type parameter specifies the content type of the first or “root” part. Multipart/related processing takes precedence over content-disposition. Many MIME user agents do not recognize multipart/related and treat these messages as multipart/mixed. To allow for this, some UAs will include the technically unnecessary Content-Disposition header in multipart/related body parts. Content-Location and Content-Base headers are defined to resolve URL references to other body parts. Both headers are valid in any message or body part. They are valid for the content heading or message heading where they occur and for its content. The Content-Location and Content-Base headers apply to headers and body parts where they occur and do not have meaning in multipart headings. The Content-Base header gives a base for relative URIs occurring in other heading fields and in HTML documents that do not have any BASE element in its HTML code. Its value must be an absolute Uniform Resource Identifier (URI). The Content-Location header contains a URL that specifies the body of that body part. The URL may be relative to a URL specified in a Content-Base header. The following example shows how these headers are used:
Content-Type: Multipart/related; boundary="boundary-content_example";
type=Text/HTML; start=example@somplace.com
;Content-Base header not allowed here
;since this is a multipart MIME object
--boundary-content_example
Part 1:
Content-Type: Text/HTML; charset=US-ASCII
Content-ID: <example@somplace.com>
Content-Location: http://www.webpage/images/the.one
; This Content-Location must contain an absolute URL,
; since no base; is valid here.
--boundary-content_example
Part 2:
Content-Type: Text/HTML; charset=US-ASCII
Content-ID: <example2@somplace.com>
Content-Location: the.one ; The Content-Base below applies to
; this relative URL
Content-Base: http://www.webpage/images/
--boundary-content_example--