Implications of outsourcing email

[MaxPageant () aol com]

Check out response.microsoft.com identifying Postfuture (www.postfuture.com) 
as the company sending transactional emails (order receipts, .NET 
confirmations) for Microsoft.

Don't know if you all know how MX records work in email, but it's possible 
for an email service provider to configure their mail server to send emails 
through their client's domain.  This makes emails you receive look like they came 
directly from the company/brand rather than their outsourcing email vendor. 
The benefit for all of us for this is it helps to authenticate opt-in messages 
as coming from the sending company rather than a fraudulent spammer trying to 
get your email.

Question: Why don't more companies do this to authenticate their commercial 
opt-in email????