DNS lookup?

How do I write my own DNS lookup to list all sub domains on a host.
Because you see i want to write an smtp server, but as i've learned, after i send mail with my smtp server, it sends mail to the destination's smtp server. but how my smtp server finds the destination's smtp server's address?
So I did a little reasearch and I found out that it's done with dns lookup, but i could not find a manual on how to do that.

Please help.
Thanks.

You need to lookup mx dns records. MX is short for Mail Exchange. These records point to the incoming smtp server for a domain.

Some operating systems have apis to lookup mx records. For example, DnsQuery is available on Windows 2000/XP. Alternatively you can use a library or just some code using sockets.

C code using sockets:
http://cvs.synchro.net/cgi-bin/viewc...bs3/mxlookup.c

DnsQuery function:
http://www.developer2.com/features/stories/34508.html

Thanks alot.