Is it possible, in e-mail, to specify that a certain body part is not included, and must be fetched from the net by the recipient? Which construct in the e-mail standards are specially defined for this?


Yes, there is a special MIME Content-Type for this,

Content-Type: Message/external-body

(A similar effect can also be achieved with an HTML-formatted message, containing some kind of code to explicitly download a certain URL. This method, however, may be less secure.)

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