I wonder if there's a way to configure courier to accept email for delivery, and return failure notifications only for specific invalid recipient, while delivering to all others?
Not for mail submitted via SMTP.
If a mail client fails to notify the user of a rejected recipient address, looks like this is a problem with the mail client.
With SMTP the client knows exactly which address is rejected, and is fully capable of either removing the address from the recipient list, or prompting the user to fix it.
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