Robert LeBlanc wrote:
Connections arriving on port 25 can be assumed to come from
servers with MX records, so that becomes a testable assumption and a
precondition for connection.
There are two things that are wrong with that statement.
1) MX records are a good idea, not an absolute requirement.
2) MX records are for determining a domain's incoming mail servers. The
problem at hand is determining whether you're correctly dealing with a
domain's outgoing mail servers.
For a SOHO mail server, you might be able to assume that the incoming
and outgoing mail servers are the same. But, that isn't necessarily
true, and it's an assumption that doesn't scale to large organizations.
