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.