I highly recommend not using this, it has a variety of weaknesses and vulnerabilities. https://bitmessage.org/forum/index.php?topic=1666.0 is a pretty thorough analysis. The biggest problem IMO is that there is no encryption when passing messages between peers, so a passive adversary can easily determine who's sending what message.

There's over 30 years of papers and academic research on anonymous mailing systems and Bitmessage incorporates not one of those concepts