I have a Linux reseller account. To access webmail on any of the hosted accounts, I go to http://www.domain.com:2095. However, I just discovered that http://webmail.domain.com works, as well. It turns out that there is an A record for "webmail" that points to the domain name, of course. What I don't understand is how the server knows to point webmail.domain.com to port 2095. Can someone explain it so that I can understand it?

Ok have a look at this link - http://docs.cpanel.net/twiki/bin/view/AllDocumentation/CpanelDocs/WebMail

study it in detail you will get answers to your questions

That goes through the proxy setup on the server. This was done as on many network custom ports are blocked.

You can use URL in following format to access respective services
cpanel.yourdomain.com
whm.yourdomain.com
webmail.yourdomain.com
webdisk.yourdomain.com

To access webmail on any of the hosted accounts, I go to http://www.domain.com:2095. However, I just discovered that http://webmail.domain.com works, as well.

Subfolder works as well yourdomain.com/webmail yourdomain.com/cpanel

Ok have a look at this link - http://docs.cpanel.net/twiki/bin/view/AllDocumentation/CpanelDocs/WebMail

study it in detail you will get answers to your questions
I'm not seeing what you are recommending. When I click on that link, all I get is a web page on how to access web mail.