Roundcube/sendmail reports error with bcc

[Robert S]

If I send a bcc message with an empty "To" field I get the following error on roundcube: "Failed to send message".  My syslog shows the following:

Feb 27 07:36:47 myserver sendmail[10483]: s1QKalkA010483: Authentication-Warning: myserver.mydomain.com.au: apache set sender to [email protected] using -f
Feb 27 07:36:47 myserver sendmail[10483]: s1QKalkA010483: [email protected], size=451, class=0, nrcpts=1, msgid=<[email protected]>, relay=apache@localhost
Feb 27 07:36:47 myserver sendmail[10483]: s1QKalkA010483: s1QKalkB010483: DSN: undisclosed-recipients:;... List:; syntax illegal for recipient addresses
Feb 27 07:36:47 myserver sm-mta[10484]: STARTTLS=server, relay=myserver [127.0.0.1], version=TLSv1/SSLv3, verify=NO, cipher=DHE-RSA-AES256-GCM-SHA384, bits=256/256
Feb 27 07:36:47 myserver sendmail[10483]: STARTTLS=client, relay=[127.0.0.1], version=TLSv1/SSLv3, verify=FAIL, cipher=DHE-RSA-AES256-GCM-SHA384, bits=256/256
Feb 27 07:36:48 myserver sm-mta[10484]: s1QKal4r010484: from=<>, size=2288, class=0, nrcpts=1, msgid=<[email protected]>, proto=ESMTP, daemon=MTA, relay=myserver [127.0.0.1]

Can somebody please explain what is going on here.  I've put a question on the sendmail list and details have been requested including a message example that causes this error (all messages cause this), and how it is submitted to sendmail (SMTP, CLI? in the latter case: which options are used)?  Perhaps someone could help out here.