If you are trying to get SpamTitan to hold all mail by setting the destination server as an unrouteable IP address, do not use 0.0.0.0
==> it will actually go back to itself, and you will get an "Loops Back to myself" bounce that will reject the message.
Don't use 0.0.0.0 as a unreachable destination server -- it goes to localhost
-- from cloudc.spamtitan.com --
telnet 0.0.0.0 25
Connected to 0.0.0.0.
Escape character is '^]'.
220 cloudc.spamtitan.com ESMTP Postfix
==> that will cause a loop back bounce.
telnet 0.0.0.4 25
==> this will make the message deferred, as intended.