Were I to have to deal with this situation I would probably handle the complexity by delivering to a local postfix instance that queues mail.
It can then forward mail either directly (I would have it try direct first, and fall back) or via one or more outbound paid services.
You can also manage its outbound rate: Postfix Configuration Parameters
This will also ease the burden on your server since the relatively expensive operation of generating the email will only happen once, not over and over (if it gets rate limited).