How to Change Webmail URL to http://mail.mydomain.com?

Assume your server's Internet IP address is 123.123.123.123 and your domain name is "mydomain.com", domain's MX is "mail.mydomain.com". As default, AA Mail Server will auto create a virtual application) with name "mail" in your IIS.

Your default Webmail address will be http://123.123.123.123/mail or http://mail.mydomain.com/mail. If you want to change Webmail URL to "http://mail.mydomain.com", you can create a text file "default.htm" in "c:\inetpub\wwwroot" folder with below content:

<script language="javascript">
window.location.replace("/mail");
</script>

There are also other ways to achieve that.
[1] You can change IIS default root folder from c:\inetpub\wwwroot folder to c:\aamailpro\web in IIS.
[2] If you host multi sites on your server, you can assign a site to AA Mail Server Webmail.
[3] If your ISP has URL forward function, you can use it to forward mail.mydomain.com to mail.mydomain.com/mail.