Thanks for stopping me before I did all the hard work!
I'm not really forging or spoofing since the email addresses belong to me as domain owner.
Is it not possible to instruct DH's SMTP server to take the FROM address and use it as the SENDER address. Of course, I am talking about the part before the domain name so there's no danger of a domain owner from spoofing an email from another domain.
I just want to use my own disposable system: it's too easy for a bot to strip out the "disposable" part of a gmail address and thus capture a nice, clean address for SPAM. Also, this doesn't really help me: I can give a sub-addressed email out on a webform say, but if the person mails me at that address and I reply from Outlook, I'm back to the same problem of being unable to send a sub-addressed email via DH.