What we do now is let you set up email addresses and forward them to your normal email address. Then programs like Outlook express let you set up accounts.
So what you would do is set your email address as:
sales@mydomain.com to forward to say your normal email address: you@yourdslcompany.com
Then in Outlook express use a filter to put all emails sent to that address into a folder. From there you create an account in Outlook and when you reply to those emails choose your sales@mydomain.com account to reply from and your customers will receive emails that appear to be from your email at your domain.
Most people dont want to have any more email addresses than they already have so this solution works pretty well and doesn't require anyone to learn a new email system since they can just forward all emails for their domain to their own normal email address.