back a few years, some mail servers (e.g. qmail) disabled the verify command to avoid address probing - and as a consequence would send bounces. Nowadays, the majority of mail servers (apart from aol :) rejects unknown users with a 5xx response to RCPT TO and thereby re-enables verification. Apart from tarpitting too many recipients, what is common practice for a server that detects verification attempts (i.e. successful rcpt followed by quit) .... ignore, blacklist, other?
Wolfgang Hamann
