Whether it is by design or accident it is just wrong.

Email, in all its glory and protocols, is full of stupid workarounds for stupid workarounds for errors in the design.

Try to write an IMAP server (or client), for example. You basically have to take a good guess at what software in what version you are talking to on the other end, so you know with errors you have to make in order for the workarounds of the other program to not mess up the communication.

It might have gotten a lot better since i last did this over 15 years ago, but i haven't checked.

I mostly do HTTP these days, and there are a few hidden gems/least used features in there, too that are still client dependent. Status code 100 is such an oddball. The client can send an "Expect: 100-continue", and the server has to send an intermediate response of "100 Continue" before the client proceeds to send the request body.

Nobody does this anymore, with the notable exception of "curl" on PUT requests. It waits patiently for 10 seconds, and without any response from the server it then proceeds to send the content anyway. So, in order to speed up a specific use case by a client, i had to implement the whole rigmarole of handling partial requests to speed up a sync script from hours to seconds. And now i have another piece of barely maintained, seldomly tested piece of code. That may or may not aggregate some workarounds over time for specific badly implemented useragents, that, for example send the Expect header, but fail when getting the correct response (either "100 Continue" or one of a number of valid error codes).

As soon as your software, or protocol, or markup language parser has to support a couple of decades of backwards compatibility, it starts to get really weird. SMTP (Simple Mail Transfer Protocol) was implemented in 1983, HTML in 1993. MIME (Multipurpose Internet Mail Extensions) was only first standardized in 1996 (around the same time that Microsoft first released Outlook). So it's fair to assume that at least some of the weird features with Microfts mail software is due to backwards compatibility. And with 3-4 decades of people trying (and failing) to fix email standards.