Linux server, it is all but instant. When sending mail to the server, there is a 40 - 45 second delay before the sent mail is accepted.
Name resolution is done via a hosts file on each machine.
It is pretty much as-is from the RedHat installation. In case it isn't already obvious, I'm a newbie at mail configuration issues. Some of these involve doing doubled reverse DNS lookups to check that the forward and reverse names are consistent. This is because the standards call for it to look up DNS MX records in preference to other types of address records.