I get a message destined for user(a)somedoamin.com, I
need to be able to
determine the host for that domain, depending on what the host is I need to
use either TCP or UDP, besides let's say,
somedomain.com resolves to
two IP addresses, I also need to try the first IP and if it fails (IP
connectivity-wise) then I try the second one.
try configuring your dns (srv records) accordingly. that should be all
you need to do.
-- juha