I think I failed to explain my problem correctly. It seems to me whatever I do for the test would be on the server side, not client side. Studying your reply, I think your solution maps domain.com to ipaddress (dotted quad) from the client side. What I need to do is have the one ip address I have for the server host iredmail and my website.
Iredmail creates an index.html in /var/www with contents
<html>
<head>
<meta HTTP-EQUIV="REFRESH" content="0; url=/mail/">
</head>
</html>
I created a /var/www2 with a different index.html, but I don't see how nginx would know which directory to use.
I was able to get nginx working with my website prior to having iredmail on the same server. Having both services on the same server is what confuses me.