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.
These goals are achieved enquiring the existing vpopmail archives (each format is supported: cdb, MySQL, LDAP, etc.) by using standard vpopmail calls, or using customized chkuser routines.
Version 2.0 - From chkusr to chkuser
Version 2.0 is a lot different from previous versions, so it deserves a more evident change in the name.
Version 2.0 has been designed with the goal to be modular, and to make more easy both adding new features to chkuser code and semplifing code update.
Patching over original qmail files is done over a few points, while the most of chkuser code remains outside, in dedicated chkuser's files.
Same for settings, that are inside a dedicated chkuser_settings.h file.
The intention is to simplify upgrading: for future chkuser releases, upgrading will require only to update chkuser specific files, leaving all the rest untouched, and changing chkuser_settings.h only if new features must be enabled.
Logging and SPAM
chkuser 2.0 has detailed logging of accepted and refused recipients and senders, allowing a deep analysis of "who's sending to whom". This can lead to more sophisticated future enhancements of anti-SPAM features.