On Tue, 2006-06-20 at 14:19 -0400, John Hinton wrote:
Every seems to be fine, but I'm constantly receiving messages in my 3 MX servers, even if mail.example.com (the less MX record) is available waiting for connections from outside... Why if my less mx record server is available the others servers are constantly receiving messages for my domain?
There is a sendmail milter available for dealing with this. Basically, it can tell the backup mailservers to look for the primary before receiving mail. This is pretty much a must if you're going to do any volume of reliable email service. It also can look ahead for a real user before receiving mail on a backup system. The milter is called milter-ahead.
http://www.milter.info/sendmail/milter-ahead/
It does cost a bit... for commercial use.. or is free for individual use.
If you mean to check for a valid smtp recipient on a different machine before accepting for forwarding, it is relatively easy to do that with MimeDefang too - along with about any other virus or spam checks you'd like: http://www.mimedefang.org/.