I feel like that's weirdly true and false.
Yes, email clients are "broken". It is "easy to see" that fixing them is the key to really useful communication.
The weird thing is that any time you get one of these "broken" mediums, its users nonetheless become adapted to it. Even the stupidest original behavior becomes a part of a common, even socially, understood medium and changing it thus becomes harder.
An old girlfriend of mine was not terribly computer literate but understood the trick of clicking on an email link in Internet Explorer, having an outlook express window come up and then copying the resulting email-address to her webmail account to send an email. Once she learned this trick, she was far less concerned about the round-about-clunkiness involved in the whole process than I would be. It was just "how computers worked" and a simpler process she didn't know would quite possibly confuse her. She probably wouldn't even know that Outlook Express was an email client itself, for example.