#1: The DNS entry for your webpage, eg
www.yoursite.com is probably hosted at your ISP, and is in their DNS record (zone file). Your e-mail is remotely hosted. So, at the company that hosts your e-mail, in their DNS (zone files) mail.yourhost.com is pointed to the IP of of your domain.
#2: As long as you don't switch hosting companies, and you keep the same IP address that mail.yourdomain.com points to, it doesn't matter where you host your mail as the company that hosts your e-mail just has to update their DNS so mail.yourdomain.com points to your current IP address at your current hosting company.
I hope that helps, it is difficult for me to explain as I am not a techno guru. But it makes sense what you are doing and isn't that complicated.
