
After you have the ability to receive emails sent to your domain email eg. sales@myname.co.nz (and tested it) you will want to set yourself up to be able to send emails from that address (and not your ISP email address)
The exact procedure for you to do this varies, depending on which email client you are using, but the basic idea is the same. In your email settings/options/setup find an option to specify or change your reply/from/email address, set this to your domain name email. If you have trouble finding this setting check your email program's help file.
Send a test email to yourself, your domain email should now show up as the 'reply address'
Note: Gmail has an option, after you create different accounts you can select "Reply from the same address the message was sent to" - this is great for managing multiple sites from one email account.