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The original qmail-smtpd accepts by default all messages, checking later for the existence of recipients. So, if the message is delivered to not existing recipients, a lot of additional system work and network traffic are generated, with multiple expensive bouncing if the sender is a fake one. chkuser has been developed with the goal to improve the acceptance SMTP phase of qmail-smtpd. qmail-smtpd patched with chkuser may check the existence of e-mail recipients immediately in the SMTP acceptance phase of a message and rejects instantly all messages not directed to existing users, avoiding additional traffic, work and messages bounced more times.