Are there any SMTP servers that deliver mail synchronously? So that if the delivery command fails, the sender gets an SMTP error? This would allow returning error messages without risk of backscatter.
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I'm not sure about *delivering* synchronously, but you can perform checks synchronously (e.g. reject nonexistent addresses or mails that fail some spam checker). What condition are you looking to error out on specifically? Postfix can probably do what you want.
See e.g. http://www.postfix.org/ADDRESS_VERIFICATION_README.html#how … which uses probe messages to verify both local and remote addresses.
