net_smtp: Clarify Bcc and sendmail semantics.

* Log warning if Bcc headers are present in DATA.
* Add sendmail client instructions to README.

README.rst

@@ -411,6 +411,35 @@ What format does the BBS use to store email?
 The BBS mail servers use the maildir++ format. This is similar to what software like Dovecot and Courier use by default,
 although certain implementation details may differ.
 
+Does the BBS provide a sendmail binary, for submitting local mail?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+No, it does not. Consequently, you may see messages like this in your cron logs, for example:
+
+:code:`(CRON) info (No MTA installed, discarding output)`
+
+This is because cron did not detect :code:`/usr/bin/sendmail`, which is used by default to submit outgoing mail from outside of the local MTA.
+
+Installing the actual :code:`sendmail` is overkill and not recommended, since it also includes the Sendmail MTA, which will conflict with LBBS.
+However, you can install a lightweight client like :code:`ssmtp` or :code:`msmtp` (a more actively maintained variant) to do this.
+You just need to ensure you install an SMTP client consistent with the Sendmail interface, so that programs expecting sendmail
+will work properly.
+
+If you install msmtp, be sure to `configure it system-wide <https://marlam.de/msmtp/msmtp.html#A-system-wide-configuration-file>`_.
+
+The below is a good default :code:`/etc/msmtprc` for most systems::
+
+   account default
+   host 127.0.0.1
+   port 25
+   from [email protected]
+   tls off
+   logfile /var/log/msmtp.log
+
+Make sure to substitute the default "from" address with something appropriate for your server.
+
+Then, you can symlink msmtp to sendmail, and things should "just work": :code:`ln -s /usr/bin/msmtp /usr/sbin/sendmail`.
+
 Does the BBS provide any kind of webmail access?
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 You can use `wssmail <https://github.com/InterLinked1/wssmail>`_, a fast and efficient webmail client designed with the BBS's mail server in mind (but may be used with any mail server).

nets/net_smtp.c

@@ -2673,6 +2673,27 @@ static int handle_data(struct smtp_session *smtp, char *s, struct readline_data
 				}
 			} else if (!smtp->tflags.dkimsig && STARTS_WITH(s, "DKIM-Signature")) {
 				smtp->tflags.dkimsig = 1;
+			} else if (STARTS_WITH(s, "Bcc:")) {
+				/* This is unexpected, and probably not good news for whatever
+				 * is sending this message.
+				 *
+				 * According to RFC 2822 3.6.3, the sending agent is responsible
+				 * for removing the Bcc: line and just providing it using RCPT.
+				 * So, MTAs should not need to be concerned with Bcc at all,
+				 * since it won't (or shouldn't) appear in the DATA, i.e.
+				 * there should never be "Bcc" headers in messages.
+				 *
+				 * mail.c does handle this in bbs_mail_message, but that's because
+				 * this API is used by things that generate messages and include
+				 * the recipients in the message. It's not unlike the sendmail
+				 * client program which also removes Bcc lines but includes them
+				 * in the RCPT: they need to be processed *somewhere*, but that's
+				 * before it hits core MTA logic, and mail.c is here acting as
+				 * the submitter / user agent.
+				 *
+				 * So, if we see this, then somebody's mail user agent is probably
+				 * not RFC-compliant and is leaking BCC's... probably not desired. */
+				bbs_warning("Message contains a 'Bcc' header? (%s)\n", s);
 			} else if (!len) {
 				indataheaders = 0; /* CR LF on its own indicates end of headers */
 			}