I wrote sendmail as a part-time, unpaid project. It was intended to be a quick way of making Arpanet mail more accessible to people at U.C. Berkeley. The key was to forward mail between existing networks, all of which were implemented as standalone programs that were unaware that more than one network even existed. Modifying more than a tiny amount of the existing software was infeasible with only one part-time programmer. The design had to minimize the amount of existing code that needed to be modified as well as the amount of new code that needed to be written. This constraint drove most of the rest of the design principles. As it turned out, in most cases they would have been the right thing to do even if there had been a larger team available.