So what I need Postfix (or some suitable alternate) to do is:
1. Don't attempt to deliver mail directly to the recipient's SMTP server. Instead act as a proxy for the SMTP servers provided by Gmail, etc.
2. Choose an upstream SMTP server to route outgoing messages through based on recipient-based rules.
3. Rewrite the From field based on the upstream SMTP server used.
No problem. #1 is pretty trivial to solve by telling Postfix about your credentials for those services (using the smtp_sasl_auth_enable and smtp_sasl_password_maps parameters, and possibly smtp_tls_security_level to enable encryption and authentication). #2 can be done using transport_maps, and #3 using smtp_generic_maps.
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