Suppose you have a plain content part:
Header: value
Header: value
content...
In HTTP, there is no requirement to add an additional "\r\n\r\n" at the end of the content (Should newline be included in http response content length?), it uses only content-length (or chunked) to determine the size of the message.
However the question is: is it necessary for email? I can't find the exact place in the (many) rfcs related to mail that define how a "normal" content part should end.