We have an Exchange 2013 server.
I dont know when this has started, but when someone from outside my organization sends us an email with an unknown recipient, they will never know that this address doesn't exist.
I've tried from my gmail, hotmail and yahoo account to send an email to firstname.lastname@example.org, it never bounced
I can see the email not being blocked by our junk filter.
I also see the email reached our Exchange server, as per the log:
2015-10-22T13:18:29.724Z,Inbound Proxy Internal Send Connector,08D2D52D25676A4F,111,>,MAIL FROM:<email@example.com> SIZE=0 AUTH=<>,
2015-10-22T13:18:29.724Z,Inbound Proxy Internal Send Connector,08D2D52D25676A4F,112,>,RCPT TO:<firstname.lastname@example.org> ORCPT=rfc822;email@example.com,
2015-10-22T13:18:29.739Z,Inbound Proxy Internal Send Connector,08D2D52D25676A4F,113,<,250 2.1.0 Sender OK,
2015-10-22T13:18:29.739Z,Inbound Proxy Internal Send Connector,08D2D52D25676A4F,114,<,550 5.1.1 User unknown,
So I guess my Exchange server 'knows' the recipient is unknown, it's just not telling the other server.
Thanks in advance