Can an SMTP server tell the sending MTA to send to another address?

Solution 1:

No. The current RFC (5321) doesn't list any response coes that instruct the sender to re-attempt to a different address. I doubt you'd want to trust that response even if it did. If the recipient can't be reached at the domain I specified, I would ignore any instruction that told me to connect to some 3rd party MX.

Addendum SMTP was built around unreliable networks with a strict process of hand-over. If the recipient doesn't explicitly confirm receipt from my sending MTA, then the message is still my responsibility. Add to this the equivalent of HTTP 302 redirects: What if I get one half-way through my redelivery schedule, does that reset the timer? What if I get a chain of them (or a loop)? How do I structure an NDR to the sender if there's potentially several failed recipients to report? It gets messy.