> It's really a lot better to reject spam during the SMTP
If you could trust your spam detection algorithms, yes. In practice you have false positives and then rejection is fatal.
> recipient's »spam« folder, which they usually never check,
Many users might not. I always do, and I find false positives every week (Gmail). Sometimes just mailing list messages of little value, but I've also found personal messages from a friend or a job offer (and it was not a pharmacist's job)