Carl Harris’s implementation was very sound, but exhibited a kind of unnecessary complexity common to many C programmers. He treated the code as central and the data structures as support for the code. As a result, the code was beautiful but the data structure design ad-hoc and rather ugly. The first serious change I made was to add IMAP support. The payoff from these simple measures was immediate. From the beginning of the project, I got bug reports of a quality developers would kill for, often with good fixes attached. How (you may well ask) did I do that?”]
Source: http://www.catb.org/~esr/writings/cathedral-bazaar/cathedral-bazaar/ar01s06.html