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.
SHOULD - Should really be there. It's not MUST, you can ignore it but do not come crying if your email is not delivered to some of your customers ! you should have though about that before.
3. SHOULD This word, or the adjective "RECOMMENDED", mean that there
may exist valid reasons in particular circumstances to ignore a
particular item, but the full implications must be understood and
carefully weighed before choosing a different course.
Any time any document (standards or otherwise) says something is recommended, then of course you should think it through before going against the recommendation. Going from their verbiage to:
> SHOULD is effectively REQUIRED unless it conflicts with another standards requirement or you have a very specific edge case.
is a fairly big leap.
Note the use of the word "must" used twice there. Barring a sufficiently good reason and accepting the consequences, this becomes a very poorly worded "required".
The spec would have been far better starting with SHALL and then carving out the allowance for exceptions.
Those reasons can be anything. Legal, practical, technological, ideaological. You don't know. All you know is not using it is explicitly permitted.
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.
Yes it absolutely is: https://www.rfc-editor.org/rfc/rfc2119 is quite clear.
3. SHOULD This word, or the adjective "RECOMMENDED", mean that there
may exist valid reasons in particular circumstances to ignore a
particular item, but the full implications must be understood and
carefully weighed before choosing a different course.
If the server has considered fully the implications of not having a Message-ID header, then it MAY continue processing.
In general, you will find most of the Internet specifications are labelled MUST if they are required for the protocol's own state-processing (i.e. as documented), while specifications are labelled SHOULD if they are required for application state-processing in some circumstances (i.e. other users of the protocol).
That is not a rule.
In this situation the server can reject any message if it wants to, and not doing a SHOULD tests the server's patience, but it's still ultimately in the "server wanted to" category, not the "RFC was violated" category.
The RFC is a request for comments. The specific one in question is about Internet Mail.
If server implementers want their mail to be delivered these are things they SHOULD do.
That's it.
It isn't something you can give to your lawyer, and nobody cares about your opinion about what you think "should" means you can make someone else do. This is how it is.
And the line of yours I quoted is still not supported by anything.
How does Google know whether or not the sender has a valid reason? They cannot know that so for them to reject an email for it means they would reject emails that have valid reasons as well.
You and I have different definitions of "clearly"
It is not required for the protocol of one SMTP client sending one message to one SMTP server, but it is required for many Internet Mail applications to function properly.
This one for example, is where if you want to send an email to some sites, you are going to need a Message-ID, so you SHOULD add one if you're the originating mail site.
> How does Google know whether or not the sender has a valid reason?
If the Sender has a valid reason, they would have responded to the RFC (Request For Comments) telling implementers what they SHOULD do, rather than do their own thing and hope for the best!
Google knows the meaning of the word SHOULD.
> it means they would reject emails that have valid reasons as well.
No shit! They reject spam for example. And there's more than a few RFC's about that. Here's one about spam that specifically talks about using Message-ID:
The server "considers" nothing. The considerations are for the human implementers to make when building their software. And they can never presume to know why the software on the other side is working a certain way. Only that the RFC didn't make something mandatory.
The rejection isn't to be compliant with the RFC, it's a choice made by the server implementers.
I don’t care what the protocol rfc says, the client arbitrarily rejecting an email from the server for some missing unimportant header (for deduction detection?) is silly.
Yes. https://www.rfc-editor.org/rfc/rfc2821#section-6.3 refers to servers that do this and says very clearly:
These changes MUST NOT be applied by an SMTP server that
provides an intermediate relay function.
> Stop hiding behind policy and think for yourself.
Sometimes you should think for yourself, but sometimes, and friend let me tell you this is one of those times, you should take some time to read all of the things that other people have thought about a subject, especially when that subject is as big and old as email.
There is no good reason viva couldn't make a Message-ID, but there's a good reason to believe they can't handle delivery status notifications, and if they can't do that, they are causing bigger problems than just this.
“MAY This word, or the adjective "OPTIONAL", mean that an item is truly optional… An implementation which does not include a particular option MUST be prepared to interoperate with another implementation which does include the option, though perhaps with reduced functionality. In the same vein an implementation which does include a particular option MUST be prepared to interoperate with another implementation which does not include the option (except, of course, for the feature the option provides.)”
Note how it explicitly calls out interoperation with implementations that do or do not implement MAY. As a exception that proves the rule, we can reasonably assume that not interoperating with a system ignoring a SHOULD rule is a correct implementation and it is the fault of whoever is not implementing SHOULD.
The standards, to my observation, tend to lag the CVEs.
Side-note: If someone has built a reverse-database that annotates RFCs with overriding CVEs that have invalidated or rendered harmful part of the spec, I'd love to put that in my toolbox. It'd be nice-to-have in the extreme if it hasn't been created yet.
CVE classify a lot of things that have nothing to do with security.
Not having a Message-ID can cause problems for loop-detection (especially on busy netnews and mailing lists), and with reliable delivery status notification.
Dealing with these things for clients who can't read the RFC wastes memory and time which can potentially deny legitimate users access to services
> It seems that Gmail is being pedantic for no reason
Now you know that feeling is just ignorance.
That should have already happened. Google is not the "first stop".
> hard ban the sender server version until they confirm
SMTP clients do not announce their version.
Also I don't work for you, stop telling me what to do.
> A midway point that involves a doom switch is not a good option.
No shit. That's almost certainly a big part of why Google blocks messages from being transited without a Message-ID.
Is it still a strong spam signal? Hard to say. Sources disagree. But as with laws, heuristics, once added, are often sticky.
"SHOULD" is basically, if you control both sides of conversation, you can decide if it's required looking at your requirements. If you are talking between systems where you don't control both sides of conversation, you should do all "SHOULD" requirements with fail back in cases where other side won't understand you. If for reason you don't do "SHOULD" requirement, reason should be a blog article that people understand.
For example, "SHOULD" requirement would be "all deployable artifacts SHOULD be packaged in OCI container". There are cases where "SHOULD" doesn't work but those are well documented.
I’m doing some work with an email company at the moment. The company has been in the email space for decades. Holy moly email is just full of stuff like this. There is an insane amount of institutional knowledge about how email actually works - not what the specs say but what email servers need to actually do to process real emails and deal with real email clients and servers.
The rfcs try to keep up, but they’re missing a lot of details and all important context for why recommendations are as they are, and what you actually need to do to actually process real email you see in the wild. (And talk to the long tail of email software).
This conversation makes me think about cornering some of the engineers with a microphone. It’d be great to talk through the specs with them, to capture some of that missing commentary.
Note "the full implications must be understood and carefully weighed before choosing a different course". Gmail and the other big hosters have full-time spam teams who spend a lot of time weighing implications, so I assume the implications of this was weighed.
Must = external requirement
I cannot fathom how you think should* would act as a requirement in any sense of the world.