alt Hacker NewsSHOULD is a requirement. It means that you have to do it unless you know some specific reason that the requirement doesn't apply in your case. "I don't want to" is not a valid excuse, "I don't see a reason to" isn't either.
IIRC this particular rule is a SHOULD because MUAs often send messages without a Message-ID to their submission server, and the submission server adds one if necessary. https://www.rfc-editor.org/rfc/rfc6409.html#section-8.3 The SHOULD lets those messages be valid. Low-entropy devices that can't generate a good random ID are rare these days, but old devices remain in service, so the workaround is IMO justified.
Replies
> "I don't want to" is not a valid excuse
for the client. If you're implementing a server, "the client SHOULD but didn't" isn't a valid excuse to reject a client either.
You can do it anyway, you might even have good reasons for it, but then you sure don't get to point at the RFC and call the client broken.
As someone who does systems engineering, the only valid requirements include the word "shall".
Should = internal target
Must = external requirement
I cannot fathom how you think should* would act as a requirement in any sense of the world.
The original email RFC is also completely unaware of how bad spam is. Sure it might mention it but it's not really AWARE of the problem. The truth is, companies like Google, Microsoft and a few others have de-facto adjusted the minimum requirements for an email. Signing, anti-spam-agreements, etc.. are the true standard if you want an email to get from point a to b. (none of which are going to be REQUIRED in the RFC)
> SHOULD is a requirement.
I once had a job where reading standards documents was my bread and butter.
SHOULD is not a requirement. It is a recommendation. For requirements they use SHALL.
My team was writing code that was safety related. Bad bugs could mean lives lost. We happily ignored a lot of SHOULDs and were open about it. We did it not because we had a good reason, but because it was convenient. We never justified it. Before our code could be released, everything was audited by a 3rd party auditor.
It's totally fine to ignore SHOULD.