Wayne Allen has been the maintainer since November 2012.
Postie was originally developed by John Blade followed by Dirk Elmendork then Rob Felty.
As far as I can tell there once was a guy named John Blade. He took some of the original wp-mail.php code and started hacking away on it. He actually got pretty far. About the time I discovered WordPress and his little hack – called WP-Mail at the time – he went on a vacation or something. There were some problems with the script, and it was missing some features I wanted. I hacked away at it and got it into a place where it did what I wanted. I started posting about it since I figured other people might want the features.
— Dirk Elmendork
A short history
June 2004 – First public release (WP-Mail) by John Blade
August 2004 0.2 released
September 2004 0.3 released
January 2005 0.311 released
March 2005 0.312 released
April 2005 0.312.2 released as “Postie”, now maintained by Dirk Elmendork
July 2008 1.1.2 released, now maintained by Robert Felty
April 2009 1.2 released
August 2009 1.3 released
April 2010 1.4 released
November 2012 1.4.5 released, now maintained by Wayne Allen
April 2013 1.5 released
September 2014 1.6 released
July 2015 1.7 released
October 2016 1.8 released
June 2017 1.9 released