After much debate and several emails from developers a debug flag has been added to blackhole.
You can enable debug using command line options Configuration options or via the configuration file Configuration file example.
Enabling the debug flag will log all incoming and outgoing commands to the blackhole log file as well as all data sent after a DATA command.
Each connection/email gets it’s own unique identifier, this indentifier uses only hexidecimal characters and is 10 character in length, which means at some point the identifier will be reused.
2013-04-14 07:30:30,788 - Connection from '127.0.0.1'
2013-04-14 07:30:31,813 - [9AF260AB7F] RECV: HELO
2013-04-14 07:30:31,814 - [9AF260AB7F] SEND: 250 2.5.0 OK, done
2013-04-14 07:30:35,074 - [9AF260AB7F] RECV: MAIL FROM:<1@1.com>
2013-04-14 07:30:35,075 - [9AF260AB7F] SEND: 250 2.5.0 OK, done
2013-04-14 07:30:38,402 - [9AF260AB7F] RECV: RCPT TO:<2@2.com>
2013-04-14 07:30:38,402 - [9AF260AB7F] SEND: 250 2.5.0 OK, done
2013-04-14 07:30:40,573 - [9AF260AB7F] RECV: DATA
2013-04-14 07:30:40,573 - [9AF260AB7F] SEND: 354 3.5.4 Start mail input; end with <CRLF>.<CRLF>
2013-04-14 07:30:47,143 - [9AF260AB7F] RECV: From: <1@1.com>
2013-04-14 07:30:52,181 - [9AF260AB7F] RECV: To: <2@2.com>
2013-04-14 07:30:54,647 - [9AF260AB7F] RECV: Subject: Testing debug
2013-04-14 07:30:56,550 - [9AF260AB7F] RECV: Hi 2@2.com
2013-04-14 07:30:57,372 - [9AF260AB7F] RECV: This is a debug message
2013-04-14 07:30:58,286 - [9AF260AB7F] RECV: .
2013-04-14 07:30:58,287 - [9AF260AB7F] SEND: 250 2.5.0 OK, done
2013-04-14 07:31:04,023 - [9AF260AB7F] RECV: QUIT
2013-04-14 07:31:04,024 - [9AF260AB7F] SEND: 221 2.2.1 Thank you for speaking to me
2013-04-14 07:31:04,024 - Closing