Re: sender rewrining advice

At some point in the future I would like to secure host b more, so authenticated users can only send out email with their assigned address.

 I'm aware that such is done by some MTAs.  I've wondered about doing that with Sendmail.  But then I realized that users were authenticating, thus I would have a good idea (but no guarantee) who, or at least which account, was being used to abuse things.  I've not needed to actually go down this path (yet).

I think this is a selling point of services like sendgrid and the likes. There are even banks using such services. So I assume they check this, otherwise it would be very easy for scammers to send out phishing emails.
Since I am thinking of developing/adding a business to consumer service, I am getting a little more interested in this.