Tonino was able to do a fast analysis of the problem.
Server: Qmail with Tonino's check user patch.
When an email sender identifying itself as SMTPSVC(6.0.3790.1830)
sends an email to multiple rcpt's, where some of the rcpt are
valid and some are invalid, not all the emails to valid rcpts
are delivered and the sender reports an incorrect list of
Problem due to Outlook not fully supporting SMTP protocol.
Sender is probably using ISA server (Microsoft integrated firewall)
that declares itself to be SMTPSVC(6.0.3790.1830) and Outlook
as the email client.
The SMTPSVC acts as a proxy, passing the SMTP session results
back to the email client.
Sender is probably using Microsoft Outlook.
Outlook is known to stop processing the "mail to:" rcpts
when it receives a "invalid user" response. After that
failure response Outlook will fail and not continue to
send the remaining rcpt's.
Hope that clears things up,