DNS/mailserver questions

[David P. Schwartz]

I have some info on DNS, but it focuses on setting up the entries on the
server so email can get routed to the mailserver.  It's not clear enough to
figure out how to poll a DNS server to get the info I need.  There are a
couple dozen specific messages you can send to a DNS server to resolve
addressing needs.  I think I need the MX message for both of these, but I'm
not clear what else.

Given some email address, say, abc@yahoo.com, how does one go about finding
the mail server's name or IP address so somebody doesn't have to bother
entering the mail server's host name, ie., "pop.mail.yahoo.com", in order to
login and read POP mail for user "abc"?

Also, given an email address, what sequence of requests are needed to get from
a domain like "yahoo.com" to a mailserver's MX name/address in order to drop
mail directly into it's mouth (rather than using an SMTP server or relay)?  Is
an MX request sufficient?

I think I need the IP address of at least one known DNS server.  This can
probably be obtained from the user's environment somewhere, but maybe some
known stable names could be used to do a lookup.  Given this, what sort of DNS
requests would I make (at the programmatic level) to get the needed info?

Thanks
-David