I got the same opinion as wingwp.
Reason:
Although all the webmail are address with your domain name (e.g. webmail.domain.com), but it's actually a redirect to your web hosting services provider's company's mail servers.
That's the reason all the service providers restricted about spamming. Although u send a mail with ur company domain name (e.g
name@domain.com), but the email service hosting company like hotmail.com will detect the email is sent from your service provider's mail server and blacklisted it.
Maybe my reason is kinda out of topic but what did i mean is the email actually not spending any bandwitdh from your hosting. I'm not really sure with the comment, but it's just my opinion.
As I said earlier, better if you ask ur service provider about the enquiry as some service provider provides different services and features.
hash