That sounds pretty reasonable for a subset of uses. In fact, it's probably possible to route all incoming mail pointed at
someAddress@domain.com
to silently go to
someAddress@sub.domain.com
So your service could, at the end of the day, be more flexible than I first imagined when I saw your reply. This sounds like something pretty amazing, frankly!
For an easy example, consider craigslist temp emails. With your service, I could generate those email addresses and have them end in @sub.domain.com. Your service would see any email sent to those addresses and then forward the reply along to S3/webhook/etc. I could then grab the contents and, if appropriate, forward those along to the actual user. This is just an obvious example that is pretty painful to set up manually, but would be pretty easy for a site to implement if it hooked into your service!