The SMTP client may be a relay point. In which case it is REQUIRED by
current RFCs to originate a bounce message to the 'reverse-path' that
may well be forged.
Explain to me why that relay couldn't do its own SMTP-level rejection?
Anyway, misbehaviour on the part of others does not free us of the
obligation to behave well. Even if that relay would spew bounces at
random strangers, _you_ are still obliged not to.
The SMTP client may be a virus or virus writer. In which case the 550,
message refuse code gives information that the virus writer should
If the virus writer wants that information, he can easily get his own
copies of the top ten virus filters.