A good behaving spam filter should either returns 5xx/4xx during an SMTP transactions or send a bounce. Returning 250 and then dropping a message without a boun

A good behaving spam filter should either returns 5xx/4xx during an SMTP transactions or send a bounce. Returning 250 and then dropping a message without a bounce is a very bad idea but luckily it is not very common - no known to me opensource MTA does this by default and some make it hard to configure this (mis)behavior even if an administrator wants it. Unfortunately email is very centralized nowadays and companies known for bad behavior in general (and deleting emails without telling anyone in particular) now control too many mailboxes.