Oh, and also ... please consider that some firewalls try to discern
whether the connection on port 25 is from a mail server or from Telnet.
While I mourn the simplicity of manual debugging of such sites, it
remains that: the fact that you can't TELNET HOST.DOMAIN 25 doesn't mean
that there's no mail service there.
Making a network connection using the application "telnet" vs the
application "sendmail" (or whatever MTA one uses) seems to be the
same when doing a tcpdump on the data. I am not sure how a firewall
would know -- purely at the network layer -- what the other side's
application was/is that initiated the connection. Yes, the other end
could try and connect back to the host, but there is no 2 way traffic
as the 3way handshake is not completing and I dont see any other
traffic coming back from that host attempting to discern any info.
