The usefulness of an individual's saved e-mail archive depends on the static and immutable nature of the content. We need our archives to serve as reliable extensions of our memories and as accurate records of the decisions, thoughts, and personal expressions of our correspondents. To the extend that modern e-mail protocols allow saved posts to incorporate content downloaded from the Internet at the time the message is (re)displayed or to execute programs that behave differently over time, they are undermining these critically important archival functions and wasting the time and effort that we put into curating our e-mail archives.
If Google's "Accelerated Mobile Pages in Gmail" project is successful, we're soon going to be receiving much more unarchivable e-mail. Thanks, fools!
“Email Is Your Electronic Memory”
Bron Gondwana, FastMail Blog, February 14, 2018
Alas, even without the distortions introduced by dynamic content, most e-mail messages go through so many filters, transducers, ad inserters, ad removers, link rewriters, and formatters that it is rare for the content provided by the sender to reach the receiver intact anyway.