'The only "must not" in 8.8 is the prohibition of changing the destination local parts.'
That's not even true. If anyone cares about this they should just read the texts. I'm certainly not going to argue this any further. We're looking at the same page and seeing different words.
It actually says this:
"However, only addresses, local-parts, or domains that match specific local MSA configuration settings should be altered"
In other words, the opposite of what you claim. You can keep banging on this if you find it useful, but I'm not going to read any more of your comments.
According to RFC2119, which defines these terms, "should" and "should not" are recommendations with the caveat that individuals not following said recommendations need to understand the full implications what they are doing.
On the other hand, the words "must" and "must not" specify absolute requirements and prohibitions of the standard, respectively.
I think the confusion comes from parsing the poorly worded "only X should be altered" which doesn't really make it clear how the "should" applies.
The clear wording would be "... should only alter X" which makes it clear that altering things other than X needs be be done with full understanding of the implications.
I know nothing about these RFCs, but this seems trivial to clear up with a reference to the text. Here's RFC6409 section 8.8: https://tools.ietf.org/html/rfc6409#section-8.8. There is exactly one "MUST NOT" and it looks like it only prohibits what your parent said it does.
There seems to be an idea floating around that anything at all is permitted under the RFC for MSAs unless it is specifically prohibited using the phrase "must not", regardless of context and other guidance in this RFP and the other RFPs that refer to it. Perhaps this is true. I have a sincere question for anyone holding this opinion: would Google be in compliance if their software altered the message body by adding the sentence "Google is the best search engine ever." here and there, at random? If not, why not, exactly?
In Poland it was standard to add ads as mail footers among homegrown free providers. Some still do. If they don't have sold ads they just advertise themselves.
Made me hate pretty much all footers, especially the bloated corporate ones.
That's not even true. If anyone cares about this they should just read the texts. I'm certainly not going to argue this any further. We're looking at the same page and seeing different words.
It actually says this:
"However, only addresses, local-parts, or domains that match specific local MSA configuration settings should be altered"
In other words, the opposite of what you claim. You can keep banging on this if you find it useful, but I'm not going to read any more of your comments.