[dqv]

But the RFC isn't only for senders it's also for receivers, isn't it?

That means there are two sides to the interpretation of what SHOULD NOT means. And in this case, senders have, due to experience, interpreted what Google does when someone SHOULD NOTs:

- The sender SHOULD NOT send us the same sequence again when we reply 550, if they do they MUST go on our shitlist.

Obviously it's not so binary and it takes retrying to several different recipients, but people have very good reason to interpret this SHOULD NOT as MUST NOT.

[mehrdadn]

No, that's not a sane way to interpret this RFC for the receiver either. I already answered this, so you'll have to go back to my earlier comment (this might be my last comment as I won't keep repeating myself): any system (be it Google's or anyone else's) that penalizes you equally regardless of whether the recipient's addressed existed 1 day vs. never existed is just plain trash. A sender that attempts delivery to an address that accepted their email a day ago is obviously unlikely to be a spammer; there's no justification for treating them as one. It is absolutely unreasonable to interpret the sentence this way. Just as it's unreasonable to interpret "the mailman shouldn't knock a second time when he's told the recipient has moved" as "I should never open the door for the mailman ever again if he does so".