[Exim] callout question

Hi,

During the SMTP session, when a RCPT command is sent, some mailserver sent
directly an error when the user is unknown on it, but other mailserver
doesnt' to that and just accept the recipient, and then the callout doesn't
work !!

Which are the reasons ? (for a smarthost mailserver is normal but for normal
mailserver with local user configured on it ?)

Thank you,

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