According to this forum thread, exchange 2013 has a new behaviour for recipient filtering. Apparently, mails to unknown users are only rejected after the DATA-Phase, not after RCPT TO as this was the case so far.
This would be a problem for many inbound gateway servers which rely on SMTP call-ahead probes to build up their local address cache (for example postfix address verification, milter-ahead, ...). Unfortunately, I don't have an exchange 2013 available to verify this myself, so my questions are:
- is it true that the new recipient filtering by default only rejects after DATA instead of after RCPT TO ?
- if this is true, is there any way to get the old behaviour back?