Description of Problem: The spec file has the comment: # No I'm not kidding above the line: Build-PreReq: sendmail I just modified the spec to not require sendmail and it built fine with exim, so I don't see how sendmail can be a build pre-requisite. Maybe it should require "mailerdaemon" or "/usr/lib/sendmail".
the build looks for sendmail by name. If you want, you can have a package which /provides/ sendmail, but I feel safer with this requirement.